Translation:Useless to try to escape; it's locked.
I do understand that there is no reference to or corresponding word for "it" in the french statement. Also, you mentioned that there is no subject in the expression to which I agree. Contrary to the notion that it is improper to use "it is useless" in place of "useless" when making the translation to English, it is a legitimate form of its usage, particularly in such circumstances.
Imagine being asked to complete the statement, "The french expression Inutile d'essayer de t'échapper, c'est fermé à clé means, " where you' are to provide the meaning. It would seem rather compelling to add, "It is useless trying to escape, it is locked" rather than "Useless to try to escape; it's locked" as the answer.
The french expression "Inutile d'essayer de t'échapper, c'est fermé à clé" means, "Useless to try to escape; it's locked" vs
The french expression "Inutile d'essayer de t'échapper, c'est fermé à clé" means, "It is useless to try to escape; it's locked"
For further reference : https://dictionary.cambridge.org/grammar/british-grammar/it
Grammatically correct sentences (clauses) in English have a subject and a finite verb. Though we often do write and speak using phrases (without a verb), this one sounds wrong to me. Maybe the French are happy to omit the finite verb? (The finite verb in "it is locked" is a secondary one and could stand alone as a sentence.) If we changed the word order to "It is locked and (therefore) [it is] useless trying to escape" the second "is" is implied; but it still sounds a bit odd without it.
Your last proposed sentence "It is locked and (therefore) [it is ] usless trying to escape" sounds odd because it is incorrect. The second "it is" is not optional. The subject "it" of the first clause is different from the subject "it" of the second clause. It will become obvious to you if you substitute the first "it" with a noun, e.g. "the house is locked and useless trying to escape".
"Trying to escape is useless, it's locked."
This is a rather better translation than Duo's, because it acknowledges the absence of "It is" before "useless." Indeed, I am surprised that this translation has been rejected, because it accords so well with the spirit of the French construction...(IMO)
t’ is short for te when using the verb escape in its reflexive form s’échapper (de) e.g. to escape (from prison). Compare the reflexive English verbs to free oneself or to extricate oneself. There is also a non-reflexive form échapper (à), which I think would be used when escaping from a person or e.g. a lion. Notice échapper au lion, not échapper du lion.
In English, you can say Useless to try escaping OR Useless to try to escape. DUO accepts only the infinitive and not present participle. Clearly not a native speaker doing that. It's similar to saying that "I like to go to the movies" is correct but "I like going to the movies" is not correct!!!! Very poor
Since there is no context for this sentence, would "de vous échapper", "de nous échapper" or "de s'échapper"? It is unclear whom this is addressed to: is it one person trying to escape (te), are we talking to more than one person (vous), are we trying to escape as a group (nous, on), or is this just a general statement about the security of the place we are trying to escape from (on = people in general)?