You can select a distr. channel only in conjunction with a sales org.

What causes this issue?

You entered a distribution channel without entering a sales
organization.
You can only select plantc via the sales organization or the
distribution chain (i.e., sales organization and distribution channel)

How to fix this error?

Enter a sales organization via which the plants to be selected are
supplied.

Error message extract from SAP system. Copyright SAP SE.