Requested function not allowed (internal error)

What causes this issue?

The function you wish to use (settlement accounting, extension) is not
allowed because changes to the rebate arrangement are not permitted.

System Response

The function cannot be performed.

How to fix this error?

As a rule, this situation is due to an internal program error, as the
function should not have been offered in the first place. Contact your
system administrator.
The relevant function has the (internal) identifier &V1&.

Error message extract from SAP system. Copyright SAP SE.