It is not possible to post the saved reversal document (document no. &1)

What causes this issue?

You are attempting to post a saved reversal document to the cash
journal.

System Response

The document cannot be updated.

How to fix this error?

Delete the reversal document and then reverse the document again.
Generally speaking, a reversal document in the cash journal should not
have the status "Saved" (yellow traffic light) (failed reversal without
deletion of reversal document).
During the reversal process, the system may issue a message indicating
the cause of the failed reversal. In this case, correct the cause of the
error (for example, by configuring the relevant Customizing settings).

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.