With your device connected to your computer, you may have to manually delete it. I just deleted a ton of workouts just now because there's a new issue where Training Calendar events are now going under "My Workouts". What I did was go to the actual Garmin device in your file explorer, then you'll find a "workouts" folder. In my case, I deleted all of them, but if you're looking for a specific one, it may be tricky since those .FIT files aren't easy to read (there's some online viewers if you search around). Good luck!
Since you posted in the Garmin Connect forum, I have to assume this is related to the Garmin Connect site. You should be able to just delete it. Is that not working for you?
But how to remove multiple workouts from the Garmin Connects web interface (or the app)?
Either one after the other, or if you have thousands to delete, then writing a script (or asking someone to write it for you) may help you doing it faster. If it is just a few dozens of workouts, then it is not worth of the effort.
