In Italian, I would say non puoi usare "muro" come aggettivo (literally, "you can't use wall as adjective").
Should I use the indefinite article before adjective in that case, or is the sentence correct/understood even without it?
English Language Learners Stack Exchange is a question and answer site for speakers of other languages learning English. It only takes a minute to sign up.Sign up to join this community
In general, in English any time you use a singular noun you must put either an article , a possessive adjective (e.g. "my"), or an adjective meaning "none" or "one" or a similar idea in front of it. (There may be some other adjectives that would work, though I can't think of one at the moment.)
"I saw a dog."
"I saw the dog."
"I saw your dog."
"I saw one dog."
"I saw no dog."
"I saw some dog."
WRONG: "I saw dog." No adjective.
WRONG: "I saw blue dog." "Blue" is an adjective but doesn't indicate possession or number, you still need another adjective. Like, "I saw the blue dog."