You need to retrieve in your microflow all the reservations that can overlap with the reservation the user is trying to make. For that, you can retrieve reservations from database, with a xPath.
For example, you can check if the User Starting Time is contained between the start and end time of the reservation (meaning the User reservation will start while the reservation is not ended) and same for the end time.
Try something like : (UserReservation/StartTime > reservation/StartTime and UserReservation/StartTime < reservation/EndTime) or (UserReservation/EndTime > reservation/StartTime and UserReservation/EndTime < reservation/EndTime)
this it my retrieve object in microflow