Some uncountable words like the word value should not be preceded with article. I am not sure about impact.
It has impact on the existing approaches.
It has an impact on the existing approaches.
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
You should use "an" because "impact" is a noun and is singular.
With a verb, you don't need and should not use "an":
It affects the existing approaches.
and with a plural noun, you should not use "an" but you may want some other word:
It has impacts on the existing approaches.
Is grammatical but, to me at least, it reads a little oddly and you might want to put "some" or "many" or "a few" between "has" and "impacts".