Order &1 is not assigned to material &2

What causes this issue?

Order &v1& cannot be assigned to material &v2&, as it is already
assigned to another material.

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?

Assign another order to the material. Find an order that can be
assigned the material or create a new order.

Error message extract from SAP system. Copyright SAP SE.