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?
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You must use the indefinite article in the sentence You can't use wall as an adjective. It's ungrammatical in English without the article. Of course the sentence will be understood without the article, but it's incorrect without it.
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."