Amount reduced in invoice is now & &

What causes this issue?

Your change has resulted in the system changing the manual invoice
reduction amount to <ZG>&V1& &V2&</>. The previous invoice reduction
amount was <ZG>&V3& &V2&</>.
INCLUDE MM_IV_MELDUNGEN OBJECT DOKU ID TX

System Response

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

How to fix this error?

Error message extract from SAP system. Copyright SAP SE.