Production version required - Planned order &1 will not be converted

What causes this issue?

The material in the planned order requires a production version.
However, the planned order was created without using a production
version. You cannot convert the planned order.

How to fix this error?

Change the planned order and enter a production version, or change the
settings of the alternative selection in the material's MRP view.

Error message extract from SAP system. Copyright SAP SE.