Release order only possible against existing contract

What causes this issue?

You want to create a PO item referencing contract &V1&. However, the PO
already references contract &V2&. A purchase order can only reference
one contract.

System Response

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

How to fix this error?

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.