the list box to show A B C D E ect

How you are loading these letters. Are they coming from some table or file.

To resolve this you can have an Inline load at the begining of the script and that should resolve your problem.

Load * Inline [

Yourfieldname

A,

B,

C,

D,

E];

Instead of Changing the script and adding inline table, we can create a ListBox with an Expression

=Left(ITEM_NAME, 1). This would generate a listbox with single unique characters according to your item name.