Using the web interface, when on the calendar, I hit the menu to add a workout, the sidebar opens and I have the option to create a new workout but my list of workouts doesn't show up so I cannot drag workouts onto my calendar.
Maybe there is something in the workout list that prevents it from loading. Like a name or an emoji in the name. Check the console for any Javascript errors.
Just to close the loop, the issue was I had workouts unchecked in the filter list. This causes all the workouts in the sidebar to be hidden, not just the workouts on the calendar.