“a SEO expert”or“an SEO expert”.

The correct answer is “an SEO expert”.

It is not whether or not to use “a” or “an” before a word that starts with a vowel, it is whether or not to use “a” or “an” before a vowel SOUND. “S”, of course, is not a vowel but the pronunciation of that letter starts with a vowel sound, i.e. “esss” and thus it gets an “an” before it.