This means terminationdate is a column name in more than one of the three tables in your SQL Query. You need to be Specific (not ambiguous) and prefix it with the table name and a period. Hint double click on the error message in the lower pane of Query Analyzer it will take you to the line that has the error.

You will need to do this in the 'where' clause. So if it's the terminationdate column in the pcphistory table that must be null put pcphistory.terminationdate is null in the where clause.