Thanks for the information Robert 
It looks like users have reported the same (or similar) issue previously (though not lately) - e.g. Workout mode not working when joining group events
The “fix” that time was thought to do with the workout files not loading. That could be either because the files are missing/corrupt on your PC (if already downloaded) or that they can’t be downloaded (if blocked by a firewall):
If its that the files are corrupt locally, a full uninsstall and reinstall of the Zwift app should fix it.
If its a firewall/network issue then that’s a bit beyond me to help you with but others may know more - maybe check antivirus is turned off or has an exception for zwift, or check if you have parental control or filtering or anything on your router or ISP that could be blocking access to the requests. It could also have just been a single/one-off network failure if this only happened once.
I’d suggest:
- check if antirivus or similar is on and blocking
- join another workout to see if it one a one off (you don’t have to ride it, just see if it loads)
- reinstall zwift if not
- start checking network problems