CI_EKPODB is the include provided by SAP to add customer fields to purchase order items, you are trying to use a tool designed to manage such fields without actually creating some.
So if you defined your customer fields via an other append structure (e.g. ZAEKPO), move those to the CI include and remove the appended structure. If you don't have such fields, you can create/activate the structure CI_EKPODB, eventually adding a DUMMY CHAR 1 field. Of course activate the include as wrote.
Regards,
Raymond