I was given this two questions in a test. The correct is the second one. I can understand it sounds better, but why would be the first one wrong? Is there anything wrong about some regarding countability?
If you are called for a job interview, wear some appropriate clothes.
If you are called for a job interview, wear an appropriate outfit.
I think the answers by Peter and Mohit Wadhwa might reflect what I see. I don't think either sentence in the question is wrong. To me, the difference is more of a nuance that makes the second a better fit.
People would understand or figure out the intent of the first sentence from the context. But it sounds a little off to my native ear because it is unnecessarily ambiguous and it would be more natural to just use more precise wording; words more typically associated with the situation.
"Clothing" is a collection of individual items and isn't usually the word of choice if you are talking about a coordinated outfit.
"Appropriate" has many possible contexts, even in association with a job interview, especially if you are referring to a collection of items rather than an outfit. For example, dressing in consideration of the weather, the amount of coverage in terms of modesty, clothing and shoes that facilitate activity if the interview might involve extensive walking or other exertion, etc.
An appropriate "outfit" or "attire", on the other hand, goes to the appearance and coordination of the items; how they look together. Either of those terms wouldn't preclude consideration of the weather, modesty, movement, or other characteristics, but the focus would be on "looking the part" or "dressing for success", which would be the intended meaning.
We can also add 'appropriate attire', 'outfit' is more specific and suggests that all the pieces of clothing should go together as a unit.
Provided this test is about grammar, both sentences look correct to me. The latter is probably 'better' in that it's more specific, but I cannot see anything wrong with the first sentence. Perhaps the test writer considers the some in the first sentence superfluous; you can rewrite the first sentence without it and retain the meaning.
If you are called for a job interview, wear appropriate clothes.
Perhaps the test writer prefers clothing over clothes, which also works and retains the meaning.
If you are called for a job interview, wear some appropriate clothing.
Having said that, I don't believe the first sentence to be incorrect grammatically.
wear an appropriate outfit.
presumes they entire clothing ensemble (outfit) goes together.
Your first sentence might be better as
If you are called for a job interview, wear (some) appropriate clothing.
but as others have pointed out
wear appropriate attire
might be more idiomatic since
business attire
casual attire
are usually used phrases.
In the first sentence, some sounds a bit surplus, that's why the second is considered better, although technically both are correct.
A lot of other possible options have been mentioned. I would also add:
[...] dress appropriately.
or
[...] dress accordingly.
