I can’t see any natural and understandable way to abbreviate “verrouillée” so I would rather suggest to write:

Porte fermée

Verrouillé, while comprehensible, is not the most common way of saying that a door is locked. Fermé à clé is more common. Depending on the context, porte fermée may be an adequate translation. Since it could mean either shut or locked, this is only suitable if there’s no reason why the display would indicate that the door is shut.

Conversely, if it’s clear that the display is about a door, then verrou fermé would work. While it isn’t an idiomatic way of saying that a door is locked, it’s absolutely comprehensible. Verrou enclenché (“lock engaged”) is more idiomatic but doesn’t fit, however verrou enclenc or verrou encl or verr enclenché might be acceptable. None of these abbreviations are immediately clear, but they’re guessable.

For an electronic lock, verrou activé is another possibility. It doesn’t really work for a mechanical lock.

Otherwise porte verr. or porte verrouil are decent ways of abbreviating porte verrouillée.

If the accent on the e can’t be shown then it’s best to avoid the word fermé because ferme also exists and means something completely different. Fermée misspelled as *fermee is less bad because that spelling doesn’t exist so fermée is a natural correction. In this case serrure fermee might be a better alternative to *verrou ferme.

How acceptable these are depends on where the display is (is it next to the door and unambiguously related to it or remote?) and who will see it (do random people need to understand it in an emergency or is this only for trained personnel?). I don’t think there’s a one-14-character-size-fits-all solution in French.

serrure fermée

verrou tiré

porte bouclée

porte bloquée (sens un peu différent)

Concernant “verrouillée”, l’abréviation utilisée sur les claviers d’ordinateurs est “Verr.” (Comme dans “Verr. Maj.” (“Caps Lock”)).verr maj

Comme alternative, je propose “Porte barrée” qui est commun, du moins au Quebec.