If all the seats for an activity are already taken, you’ll need to wait until the current group of participants completes their activities and then check back later to see if the activity will be reopened.
Each activity has a limited number of available spots, as each coach can only manage a certain number of learners at a time.
While waiting for the desired activity to become available again, you can search for other activities with open seats and enroll in one of those in the meantime.