Hi Ahmed
If you have a customer / vendor or bank ledger entry that has been applied to something or had adjustments processed against it the system will not allow it to be reversed in the register
Your solution here would be to navigate on the transaction in the GL to find the related sub ledger entry, then go to that sub ledger entry and un-apply it if it is currently applied
Next post a correcting entry and apply the correcting entry to the incorrect entry
Hope this helps
Nev