This message comes from the venue's weekly schedule: the event's start and end times do not fall inside any of the venue's Time to Start / Time to Close ranges for that weekday. In the current version this rule behaves differently depending on when it runs: on Save, a day that is switched off for the venue is treated as if no restriction applied (so it is not rejected for that reason alone), but the instant check that fires as soon as you pick the venue in the dropdown is stricter and will flag an off day as unavailable right away. Pick a time inside the venue's hours, extend the venue's hours on the venue form, or choose another venue.