Why is "Hol autót látsz?" not accepted?
Hol is always followed by a verb
Why can't there be an indeterminate article before accusative in this clause?
The indefinite article is under-, the definite article is overused in Hungarian compared to English. You can mostly omit "egy", it's implied here. You can put it in if you really want to.
“Merre látsz egy autót?” Why isn’t it good in this case?