Gramarly's automatic spell/grammer checker says there should be an "a" here.
Dear teachers.

I wrote the following text:

In Afghanistan, if you have AN un-Islamic beard you can go to jail.

And the Grammarly suggested that I replace "an" with "a".....I am confused.

Question

Gramarly's automatic spell/grammer checker says there should be an "a" here.
Dear teachers.

I wrote the following text:

In Afghanistan, if you have AN un-Islamic beard you can go to jail.

And the Grammarly suggested that I replace "an" with "a".....I am confused.

Answer

"An" is correct

Answer

@Andrew Goddard: Thank you, sir.

Answer

Grammarly is useless.

The rule is that if the noun (or, as in this case an adjective that precedes a noun) starts with a vowel sound, always use 'an,' rather than 'a.'  Quite clearly any adjective that begins with the prefix "un," begins with a vowel sound.