The key does not go into the lock.
ser-ra-TU-ra — four syllables, stress on the third.
Contact the host immediately if you cannot enter the apartment. This is an urgent situation that requires prompt resolution.
'Non entra nella serratura' = does not go into the lock. 'Serratura' = lock. 'Chiave' = key. This can happen if the wrong key is used, the key is inserted upside down, or the lock is faulty. Always describe the exact symptom to help the host diagnose remotely.
La chiave gira ma la porta non si apre.
The key turns but the door does not open.
A different failure mode — the bolt may be stuck
Non riesco ad aprire la porta. Cosa faccio?
I cannot open the door. What should I do?
A general distress call if no specific problem is identified
Sto usando la chiave giusta?
Am I using the right key?
Ask if there are multiple keys and you may be using the wrong one
Italian apartment sets typically include two or more keys for different doors: main building door ('portone'), apartment door, sometimes a storage room ('cantina'), and occasionally a letterbox key. Italian hosts should label each key clearly — if they do not, ask before they leave.