[b]"Help! Is this a syntax error in grammar, or am I missing something?"[/b] Alternatively, if you prefer a slightly

"Barks loudly" is correct, but tbh, in casual speech, people say "loud" all the time.

Technically a syntax error in grammar, but not a huge deal unless you’re writing formally.

If you want to nerd out, GrammarBook.com has great adverb exercises.