Selected plants are invalid for this purchas. org.

What causes this issue?

The purchasing organization of the plant differs from the purchasing
organization of the target reference plant. Assigned plant and
reference plant must belong to the same purchasing organization.

System Response

Cannot assign selected plant to target reference plant.

How to fix this error?

Assign the plant to another reference plant that belongs to the same
purchasing organization; refresh your display to the list of all
available plants to be assigned.

Procedure for System Administrators

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.