Do you have an empty room in this hotel?
このホテルに空室はありますか？ 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)