Reservations are in high demand. When one spot opens, there may be several users vying for that one spot.
More than one user can click on the available reservation at once, and the quickest one to checkout is the one who gets it.
That is what that error message confirms.
If this happens again, we suggest going back and refreshing as spots can open at any time when other users cancel - but they are scooped up extremely quickly.
The inventory is limited, we apologize for any inconvenience.