Material &1 with sales unit &2 has already been allocated

What causes this issue?

You selected one or more promotion items for which allocation should
take place.
You also included items for which allocation has already taken place.

System Response

Items for which allocation has taken place are ignored.

How to fix this error?

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.