Hi Era, I’ll suggest that “there is no apple” is more emphatic about the lack of apples in general. While “there isn’t an apple” is also similar and can be used as the previous, but it places more emphasis on the absolute lack of the apple. In other words, “there is no apple; no, not even one”. And there was no article after “no” because “no” itself is a quantifier/determiner/article of sorts. However, a negative quantifier at that. To understand this better, substituting the “no” with either “an” or “some” which are also quantifiers will reveal that no other article, for example “an” as you asked, can be used in addition to them in the given sentence. Thank you.

Kindly ignore the “but” in the first paragraph ;)

There is no difference in meaning. I can’t explain why, but we never use an article (a, an, the) between “no” and a noun that follows “no.” Maybe, the “no” itself is functioning as a (negative) article. I hope that somebody who is good at technical grammar can give a better answer.

Hi Era, I’ll suggest that “there is no apple” is more emphatic about the lack of apples in general. While “there isn’t an apple” is also similar and can be used as the previous, but it places more emphasis on the absolute lack of the apple. In other words, “there is no apple; no, not even one”. And there was no article after “no” because “no” itself is a quantifier/determiner/article of sorts. However, a negative quantifier at that. To understand this better, substituting the “no” with either “an” or “some” which are also quantifiers will reveal that no other article, for example “an” as you asked, can be used in addition to them in the given sentence. Thank you.