Distribution channels deviate from each other. &1 and/or &2 (reference)

What causes this issue?

The distribution channel in the reference document is not the same as
the distribution channel in the target document.

How to fix this error?

If you want to add to an existing document by referring to another
document, select a reference document which has the same distribution
channel as the target document.
If you want to create a document with reference, make sure you are
using the correct distribution channel and document number.

Error message extract from SAP system. Copyright SAP SE.