I have worked with HTML in the past, but am new to XML. I think my code is mostly correct however I cannot figure out how to fix the remaining errors. I think my brain is stuck on HTML mode. Any help would be great. This form just has to validate.
Here is the .XML code

<!DOCTYPE League SYSTEM "teams.dtd"
[
<!ENTITY Tigers SYSTEM "Tigers.bmp" NDATA Graphics>
<!ENTITY Raiders SYSTEM "Raiders.bmp" NDATA Graphics>
<!ENTITY Storm SYSTEM "Storm.bmp" NDATA Graphics>
]>
<League>
<League LName="Eastern Developmental League">
   <Team>
      <Team_Name>Tigers</Team_Name>
      <City>Toledo</City>
      <Logo Source="Tigers"/>
      <Players>
         <Player Position="Centers" PPG="21.2" RPG="11.1">Lionel Diggs</Player>
         <Player Position="Forward" PPG="18.9" RPG="8.4">Andrew Case</Player>
         <Player Position="Forward" PPG="14.4" RPG="5.0">Casey Lewis</Player>
         <Player Position="Guard" PPG="15.4" RPG="3.1" Assists="9.4">Andrew Case</Player>
         <Player Position="Guard" PPG="7.2" RPG="2.5" Assists="3.9">Ray Kee</Player>
      </Players>
   </Team>
   <Team>
      <Team_Name>Raiders</Team_Name>
      <City>Central City</City>
      <Logo Source="Raiders"/>
      <Players>
         <Player Position="Center" PPG="17.2" RPG="14.2">David Thesus</Player>
         <Player Position="Forward" PPG="28.3" RPG="7.0">Troy Thompson</Player>
         <Player Position="Forward" PPG="10.4" RPG="5.3">Gary Coles</Player>
         <Player Position="Guard" PPG="19.2" RPG="5.2" Assists="4.4">Michael Burns</Player>
         <Player Position="Guard" PPG="5.1" RPG="2.1" Assists="12.9">Patrick Dawson</Player>
      </Players>
   </Team>
   <Team>
      <Team_Name>Lightning</Team_Name>
      <City>Philadelphia</City>
      <Players>
         <Player Position="Center" PPG="12.2" RPG="14.2">Tim White</Player>
         <Player Position="Forward" PPG="21.7" RPG="7.8">Michael Keyes</Player>
         <Player Position="Forward" PPG="8.4" RPG="9.4">Bobby Phillips</Player>
         <Player Position="Guard" PPG="13.1" RPG="1.4" Assists="10.3">David Rice</Player>
         <Player Position="Guard" PPG="10.2" RPG="4.1" Assists="8.4">Elroy Watson</Player>
      </Players>
   </Team>
   <Team>
      <Team_Name>Hoopsters</Team_Name>
      <City>Hill County</City>
      <Players>
         <Player Position="Center" PPG="36.8" RPG="14.8">Tim Brubeck</Player>
         <Player Position="Forward" PPG="13.9" RPG="7.2">Steve True</Player>
         <Player Position="Forward" PPG="5.2" RPG="7.9">David Dannon</Player>
         <Player Position="Guard" PPG="12.4" RPG="5.8" Assists="7.3">Kevin Cleveland</Player>
         <Player Position="Guard" PPG="9.5" RPG="6.2" Assists="12.3">Doug Kaufman</Player>
      </Players>
   </Team>
   <Team>
      <Team_Name>Jaguars</Team_Name>
      <City>Jersey City</City>
      <Players>
         <Player Position="Center" PPG="9.4" RPG="15.1">Scott Plank</Player>
         <Player Position="Forward" PPG="24.1" RPG="8.1">Bill Avidman</Player>
         <Player Position="Forward" PPG="10.2" RPG="8.7">Dan Smith</Player>
         <Player Position="Guard" PPG="24.8" RPG="2.9" Assists="9.7">Brett Dickson</Player>
         <Player Position="Guard" PPG="8.1" RPG="6.7" Assists="8.8">Paul Thompson</Player>
      </Players>
   </Team>
   <Team>
      <Team_Name>Storm</Team_Name>
      <City>Rockaway</City>
      <Logo Source="Storm"/>
      <Players>
         <Player Position="Center" PPG="21.4" RPG="14.8">Tom Jay</Player>
         <Player Position="Forward" PPG="16.8" RPG="8.7">Alan Jackson</Player>
         <Player Position="Forward" PPG="8.4" RPG="7.8">Alan Jones</Player>
         <Player Position="Guard" PPG="19.2" RPG="1.8" Assists="14.3">Kevin Browne</Player>
         <Player Position="Guard" PPG="14.8" RPG="5.1" Assists="7.2">Ian Larson</Player>
         <Player Position="Center"></Player>
      </Players>
   </Team>
</League>

Here is the .DTD Code

<!ELEMENT League (Team+)>
<!ATTLIST League LName CDATA #IMPLIED>
        <!ELEMENT Team (Team_Name, City, Logo, Players+)>
                <!ELEMENT Team_Name (#PCDATA)>
                <!ELEMENT City (#PCDATA)>
                <!ELEMENT Logo EMPTY>
                <!ATTLIST Logo Source CDATA #REQUIRED>
                <!ELEMENT Players (Player, Player, Player, Player, Player)>
                        <!ELEMENT Player (#PCDATA)>
                                <!ATTLIST Player Position CDATA #REQUIRED>
                                <!ATTLIST Player PPG CDATA #IMPLIED>
                                <!ATTLIST Player RPG CDATA #IMPLIED>
                                <!ATTLIST Player Assists CDATA #IMPLIED>
<!NOTATION BMP SYSTEM "image/bmp">

ERRORS

Description: The notation "Graphics" must be declared when referenced in the unparsed entity declaration for "Storm".

Description: The notation "Graphics" must be declared when referenced in the unparsed entity declaration for "Tigers".

Description: The notation "Graphics" must be declared when referenced in the unparsed entity declaration for "Raiders".

Description: Unexpected element "Players". The content of the parent element type must match "(Team_Name,City,Logo,Players+)".

Description: Unexpected element "Players". The content of the parent element type must match "(Team_Name,City,Logo,Players+)".
Description: Unexpected element "Players". The content of the parent element type must match "(Team_Name,City,Logo,Players+)".
Description: Unexpected element "Player". The content of the parent element type must match "(Player,Player,Player,Player,Player)".
Description: Unexpected element "League". The content of the parent element type must match "(Team)+".

Description: XML document structures must start and end within the same entity.

in team strom 6 player define is wrong

dtd discribe 5 player

in team Lightning Hoopsters Jaguars
tag logo is not declared

you in dtd make ? so discribed 0 or 1

<!ELEMENT League (Team+)>
<!ATTLIST League LName CDATA #IMPLIED>
        <!ELEMENT Team (Team_Name, City, Logo?, Players+)>
                <!ELEMENT Team_Name (#PCDATA)>
                <!ELEMENT City (#PCDATA)>
                <!ELEMENT Logo EMPTY>
                <!ATTLIST Logo Source CDATA #REQUIRED>
                <!ELEMENT Players (Player)+>
                        <!ELEMENT Player (#PCDATA)>
                                <!ATTLIST Player Position CDATA #REQUIRED>
                                <!ATTLIST Player PPG CDATA #IMPLIED>
                                <!ATTLIST Player RPG CDATA #IMPLIED>
                                <!ATTLIST Player Assists CDATA #IMPLIED>
                                <!NOTATION BMP SYSTEM "image/bmp">

This will fix what you have shown:
1. First the <!NOTATION ....> you have listed as:
<!NOTATION BMP SYSTEM "image/bmp"> - is okay but to use it was not correct in the XML.
2. Correct the three entries in the XML to end as shown below:
<!ENTITY Tigers SYSTEM "Tigers.bmp" NDATA Graphics>

to

<!ENTITY Tigers SYSTEM "Tigers.bmp" NDATA BMP>

  1. This will validate but does Position have some limited values?
    <!ATTLIST Player Position CDATA #REQUIRED>
    if so, you can do something like this:
    <!ATTLIST Player Position (Center | Guard | Forward) #REQUIRED>

Also this line:
<!ELEMENT Team (Team_Name, City, Logo, Players+)>

Should be:
<!ELEMENT Team (Team_Name, City, Logo?, Players)+>
which makes the Logo optional and the entire Team element to occur "at least once"

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