this code should send the id to the next form in the same html page
                         <a href="newtestpage.php?id=<?php echo $post['C_ID'];?> #editEmployeeModal" class="edit" id ="<?php echo $post['C_ID'];?>" data-toggle="modal">
                         <i class="material-icons" data-toggle="tooltip" title="Edit">&#xE254;</i></a>

and this code recieve the id and fill the data in the input box

<div id="editEmployeeModal" class="modal fade">
            <div class="modal-dialog">
            <?php
      if (isset($_GET['id']))
             if (is_numeric($_GET['id'])){              
                    $id = $_GET['id'];         
                    $Query = $sql = "SELECT * FROM courses WHERE C_ID='".$id."'" ;
                    $CrPost = mysqli_query($db,$Query) Or die($Query);
                    $post = mysqli_fetch_array($CrPost,MYSQLI_ASSOC);   }
    ?>
                <div class="modal-content">
                    <form class="login100-form validate-form" action="Insert_Course.php" method="post" style="width: 100%;">
                        <div class="modal-header">   
                            <h4 class="modal-title" style="width: 100%; text-align: center;">Edit Course</h4>
                            <button type="button" class="close" data-dismiss="modal" aria-hidden="true">×</button>
                        </div>
                        <div class="modal-body wrap-input100 validate-input">  
                            <div class="form-group wrap-input100 validate-input">
                                <input class="input100" type="text" name="C_ID" value="<?php echo $post['C_ID'];?>">
                                <span class="focus-input100"></span>
                                <span class="symbol-input100">
                                <i class="fa fa-mobile" aria-hidden="true"></i>
                                </span> </div>

                        but the input is empty... whats wrong?help please!!!

Use

$id = filter_input(INPUT_GET, 'id', FILTER_VALIDATE_INT);
if($id !== NULL){
    ...
}

instead of lines 4,5,6, use prepared statement intead of direct pass variable (line 7) to prevent from SQL injection.
White space in GET parameter can be damage your variable <a href="newtestpage.php?id=<?php echo $post['C_ID'];?> #editEmployeeModal" class="edit" id ="<?php echo $post['C_ID'];?>" data-toggle="modal">
?id=<?php echo $post['C_ID'];?> #editEmployeeModal