A different source of supply is the fixed source for this date

What causes this issue?

The <GL><GL:source_list>source list</> specifies a different <GL>
<GL:source_of_supply>source of supply</> (supplier, outline agreement,
or plant) for the material for this date than the supplier entered. This
means that the material is to be procured only from the source shown in
the source list at this point in time.
The source has the number:
&V1& &V2& &V3& &V4&

How to fix this error?

Check the source list to establish whether the fixed source represents a
feasible procurement option for this purchase order:

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

<DS:TRAN.ME03>Display source list</>

Error message extract from SAP system. Copyright SAP SE.