Assignment & & already includes the billed quantity

What causes this issue?

You can only make one assignment for transferring the quantity billed
per PA transfer structure for order and project settlement. This
assignment (&V1& &V2&) has already been found for PA transfer structure
&V3&. Consequently, this flag cannot be activated for any other
entries.

How to fix this error?

Set the flag for only one assignment line in the PA transfer structure.

Error message extract from SAP system. Copyright SAP SE.