To tag a (Single) Reservation as a No-show, locate the Reservation on the Bookings screen and tap to open it

Scroll down to the Actions list and tap on Didn't Show, followed by Yes, Didn't show

Tap Done

An orange No-show tag will now appear in the Reservation tile

You can remove the no-show status by tapping on the Reservation, and tapping No-show (X) in the Status field followed by Yes,Undo