Reference document &1 posted in FM area &2

What causes this issue?

You want to reference a reservation document which was posted in
another FM area. This is not possible.
The error may occur if the document you are currently processing is
assigned to an FM area but the reservation document is not, or vice
versa.

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?

Enter a reference document which is in the same FM area as the document
you are currently working on.

Error message extract from SAP system. Copyright SAP SE.