On the proposal, open the delivery section and set Type to Venues (this option only appears when the Venues module is on and Allow Venues is switched on in both Configuration > Business: General and Catering > Settings > Proposals tab > Delivery Methods on Events). Then choose the venue in Venues Location, and optionally a Venue Event Type, Venue Layout and Venue Notes. Make sure the event date, Event Time and end time are filled in, because the availability check uses them. When you save, Sprwt verifies the venue is free before the proposal is stored.