このホテルに空室ありますか?

English Translation

Do you have an empty room in this hotel?

このホテルに空室ありますか? is grammatically more correct although the original Japanese sentence is not completely incorrect. We sometimes drop the particle は in a casual dialogue, but in that case, we don’t use the polite expression ありますか?

こちらのホテルに空室はありますか? (a polite question from a traveler to a hotel receptionist)

このホテルに空室ある? (a casual question among co-travelers who are searching rooms online)