Retro-billing is not possible.

What causes this issue?

Another user is also processing the retro-billing list.

System Response

A retro-billing cannot be created because the other user is most likely
also creating a retro-billing.

How to fix this error?

Wait until the other user is no longer blocking the retro-billing list
and try again.
Restart or regenerate the report to see if the list is still locked by
another user.

Procedure for System Administrators

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.