granting permission to users issue

first i clicked on user and groups > users

most of the time i cant seem to give a guy permission.

but for sometimes it works but other times it'll just bring me back to the main page of the admin panel.
is this a bug or is it just me? cause i tried on both my desktop and my bro's laptop.

(im sometimes able to add people in but some other times i cant and when i try again it works but at times its just frustrating,is there anyway to solve it please?)

any help or so would be greatly appreciated

thanks and regards.