if you run into that issue again, you can try refreshing your web browser and trying again.   When there are a LOT of people moving on a particular day, the TSP website has been known to overload and not let someone make a move.   So far I have generally found that 15 minutes before the cutoff, to top of the hour, can be a busy time, but I have always managed to make it take before the cutoff, even if I had to reload once or twice.  
Of course- if you make your move an hour before the time, you generally never have an issue.   
Enjoy- and good luck!