Hi Barnabas
Click on the Tab Currency/ Quantity Fields.
Specify the Reference Table Name and Reference Field Name for the Field BETRG.
BETRG is a Currency Field, So specify the Reference Field which is a Currency Key Field like WAERS and Reference Table like T001 for example.
In your Structure declare a Currency Key Field Waers and in the Tab Currency/ Quantity Fields, Give Reference table as your Structure Name and Reference Field as WAERS.
Regards
Sajid