Why a booking, invoice or month is locked

Four different things lock your books, and the app words them as locked, closed (gesperrt, abgeschlossen) or finalized (festgeschrieben). A booking is locked when it was posted, when it was exported to DATEV, when its month is closed, or when it comes from an issued invoice. The messages name the cause: read them first, then use the matching way out below. A message such as "You are not allowed to do this" is something else: it means your role lacks the right.
Posted or exported: the message says "This booking is finalized and can no longer be changed" or "The booking is locked", with the reason posted, exported or ausgangsbeleg (issued invoice). Re-opening a month does not unlock such a booking. Open the booking, choose Reverse and Book the reversal (owners, admins and accountants), then enter the correct booking anew. The reversal is dated today, so today's month must be open. See Reverse a booking (Storno).
Closed month: the message says "The month ... is closed" or "is locked". Every booking dated in that month is frozen, you cannot move a booking into it, a new invoice dated in it cannot be finalized, and with the cash basis (Istversteuerung) an income booking is also frozen when one of its payments falls in that month. A month is closed by Close month under Settings, Periods, by the month-end button, by a tax advisor's sign-off, or by a DATEV export of months that are over. Either book the correction in an open month, or ask an owner or admin to re-open the month with a reason (see Close and re-open a month).
Issued invoice: Finalize with Assign number and lock fixes the number and the content, and its income booking becomes read only with the note that corrections go through a cancellation invoice or an invoice correction. Choose Open invoice on the booking to reach the invoice, then see Cancel or correct an invoice.
In the list, a reviewed or unsorted booking that is exported or sits in a closed month shows Locked instead of its state; a posted booking keeps Posted. The label does not say which cause applies: open the booking and read the notice.
Two messages ask you to wait, not to change anything. "Another step is closing this client's books right now" (a month is being closed or a DATEV export runs) means nothing was saved: try again in a few seconds. "The filing for ... is being sealed" appears when you re-open a month whose VAT return working paper is still being sealed; try again after it finishes, and contact support if it lasts. A sealed working paper cannot be edited either.
Something else can also stop a booking from moving on: open blockers under Validation checks stop Mark reviewed and Post booking until they are released with a reason. That is not a lock. Closing a month is allowed for owners, admins, accountants and the tax advisor; re-opening only for owners and admins.