This problem happens when the corresponding unit field (CURR or QUAN) is missing from the extract structure definition, or when it references another table.
Your key figure definition contains an incorrect reference.
Check OSS notes - 335342 , 202162 for the required corrections .
335342 - RSO2: Termination when generating extract structure
202162 - R8003: IO of type KYF requires unit field
This error occurs whenever the table, in your case GLPCP, contains any currency/unit field which is maintained as a field which refers to a field in another table. If you go to the tab " Currency/Quantity fields" in SE11 for the table GLPCP, you will see that many fields, starting with HSLVT, are referred from T001 table and the referred field is WAERS.
So this table cannot be used as an extract structure for a generic datasource as generic datasource mandates that all fields should refer to the extract structure.
Therefore create a view on this table and T001 and use that view as your extract structure for the datasource.