Cannot save new reference plant &1 under class &2 in &3

What causes this issue?

New reference plant &V1& that you created under class &V2& in &V3&
contains no assigned site.
A reference plant cannot be saved if no plant has been assigned to it.

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 at least one plant to your new reference plant and save.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.