Maintain Customer Fields - Enter a data element / Billable Item Class

Good day! I am adding customer fields to a new Billable Item Class. I have added the fields into SE11 as structure CI_FKKBIXBIT_IT. I have selected Built-In Type and have populated the Data Type details. The structure has been saved and activated. However, when I try and add the Customer fields into the Billable Item Class I get an error - Enter a data element for field ZZ*. I can see all the customer fields in the selection but I cannot save the entries in the main items. Please advise on how to resolve this concern please. Thank you!