Garmin watches can only store a limited number of workouts.
If you see a message along the lines of 'workout storage full' then old workouts must be removed before the watch can load any more. You can delete workouts one by one on the watch via the training menu, but it is usually quicker to connect to a computer and just delete all files from the GARMIN/WORKOUTS directory.
Please note that workouts are different to activities – deleting activities from the watch will not help.
Furthermore, many users report that the message continues following workout deletion. All then spontaneously begins to work a few hours (or even days) later. Sometimes rebooting the watch helps.
Unfortunately this is a watch issue and there is nothing we (TrainAsONE) can do to control it. We await Garmin to address the problem - around 4 years so far...