Reverse posting key specification is missing for posting key &

What causes this issue?

Say you want to create a reverse posting for a line item with posting
key &V1&, but no reverse posting key is defined for it in the posting
key table.

System Response

The transaction cannot be carried out.

How to fix this error?

Maintain the named posting key in customizing and specify the reverse
posting key. You can then carry out the required reverse posting.

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

Error message extract from SAP system. Copyright SAP SE.