Thanks for reaching out. I can confirm there are no stale or hung sessions affecting the Catalyst Center sandbox — the environment is healthy and the reservation slots are actively in use.
Unfortunately, this is a case of high demand. The Catalyst Center sandbox is one of our more popular labs, and all concurrent slots are currently occupied by active reservations. The best approach is to keep trying, as slots do free up throughout the day as sessions expire. We also have an Always-on version of Catalyst Center if that helps.
We do appreciate your patience, and sorry for the wait.