Material number in call-off item = &, in delivery item = &

What causes this issue?

The material number in the delivery item and in the call-off item must
be the same.

System Response

It is not possible to assign this call-off item to the delivery item.

How to fix this error?

Please use another call-off item.

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.