Unable to set registration by invitation? [RESOLVED]

I currently have registration as approval only. When I go to the dashboard and try to change it to invitation, I can select it and save... but it's not "sticking". When I go back to the registration section it still shows as approval.
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Answers
Disregard... this was a permissions problem. Sorry
It's not a problem, this discussions, as long as you did it alone, will be helpful for thoses who has thé same problem ^^