The reverse key & for the posting key & is invalid

What causes this issue?

The reverse posting key is not allowed for one of the following
reasons:
account type deviates from the account type of the posting key
the reverse posting key for special sales is there, the posting key,
however, is not (or vice versa)
the reverse posting key has the same debits/credits indicator has
posting keys

How to fix this error?

Specify another reverse posting key for the posting key.

Error message extract from SAP system. Copyright SAP SE.