Hey there Kris, sorry for the inconvenience. DevNet Sandbox popularity has increased a bit faster than our capacity I'm afraid. We're currently running at peak so these errors are likely due to you trying to reserve an instance and we didn't have capacity at the moment.
We are working hard to add more capacity and stay ahead of demand. For now you can check out the Always On labs and see if they can work until reservation open back up.
PS. We are also working on making the error messages a bit clearer.