Apportionment factor '&' unknown. Correct

What causes this issue?

You have entered an apportionment factor that the system does not
recognize.

System Response

Your entry will be refused.

How to fix this error?

Enter an existing apportionment factor. You can view a list of
available apportionment factors by pressing the F4 key.

Error message extract from SAP system. Copyright SAP SE.