Sometimes a reservation is checked in prematurely or by mistake. Visual Matrix allows you to undo a check-in, but only under certain conditions. You can undo the check-in only if no transactions have been posted on the folio. If any transaction exists (even if voided), the undo is not possible. In those cases, your options are to check out the room or leave it in-house until the guest arrives.
Path: Front Office > Reservations > Search > Open the reservation