Articles are words ("a," "an," and "the") that are used to modify nouns. Here, Grammarly has caught an error in the use of the article "a":
The student has written:
Gardner had a theory about a intelligence.
There are actually two common errors in this sentence. The first error is that the singular article "a" does not agree with the uncountable noun "intelligence." Since "intelligence" cannot be counted, it does not require an article. The student could address this error by writing,
Gardner had a theory about intelligence.
The second error is a misuse of "a" in combination with a noun beginning with a vowel ("intelligence," which begins with an "i"). Nouns or adjectives beginning with vowels ("a," "e," "i", "o," and "u") require the use of "an," rather than "a," as in the following sentences:
Gardner had a theory about intelligence. An intelligent person can be many things.
The Writing Center has an overview of article use at Articles.