Material group - payment term mismatch for Fast Pay

What causes this issue?

This purchase order contains one or more Fast Pay relevant items that
have already been processed prior to the current line &V1&.
The material group &V2& to which the material in current line &V1&
belongs is different from the Fast Pay material group &V3&

System Response

You cannot post this purchase order because you cannot have one that
has Fast Pay and non-Fast Pay items mixed together.

How to fix this error?

You should change the material group of the non-Fast Pay materials to
&V3&. The purchase order can then be posted as a Fast Pay purchase
order if the other conditions are met.

Procedure for System Administrators

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

Error message extract from SAP system. Copyright SAP SE.