HELP!! How do I fix a hydrochloric acid stain on my macbook pro cover

Vinegar is an acid too. Expect it to make things worse. Aluminum is easily damaged by acids and other chemically active substances like sodium bicarbonate.

Outside of paying for a complete new top assembly at tUS$500-600, the most cost solution is an attractive sticker over the damage.

That is not a stain. The cover is Anodized aluminum, and Hydrochloric Acid likely caused a chemical reaction.

If Space Gray, the only hider is to paint it black with a Sharpie pen.

If silver-colored Aluminum, go back to your books and figure out how to put aluminum chloride into solution, [or use Ammonia, the active chemical in most Metal polishes, including Silver and Aluminum polish]. That will not return the anodizing, so the texture and sheen will never match perfectly.

Ignore it. Any attempt to repair the spot will result in more staining/discoloration. Consider it evidence of hard work and extensive use over time.

My old chemistry teacher used to say, "Ammonia: strong smell, weak base" (i.e, opposite of acid).

Any silver polish (or other metal polish) on hand?