Only customer fields may be changed in method &1 of BAdI &2

What causes this issue?

Method &v1& of BAdI &v2& changes data in a forbidden way. The following
data changes are not permitted for this method:
Changing non-customer fields
Appending data records to transferred tables or deleting data records
from tables

System Response

The remuneration document is incorrect and is not posted.

How to fix this error?

Check the method and use the correct one.

Error message extract from SAP system. Copyright SAP SE.