You may have seen other forum posts explaining we’ve had server problems recently. I think you must have been unlucky and tried to unassign during that period. I just logged in to your account and was able to unassign some NVR tests. I unassigned 6 in total just to test things out. There are 2 more NVR tests that you could unassign if you so wish.
Sorry about this. We are trying are best to keep everything running during this busy period.
All the best.
Good luck with your 11plus preparation!