Since most auto gearbox operations nowadays are controlled electronically and selection is by switches on the gear lever, rather than by mechanical movements (like a manual gearbox) I would guess that the gear lever has a mechanical fault which is stopping the button being depressed. My Volvo has a system whereby the selector cannot readily be moved out of P unless the ignition is on, which is obviously electrically controlled. I cannot believe that low fluid levels would prevent operation of the selector.
Tony - incidentally my Volvo doesn't have a dipstick for the transmission nor is there any easy way of filling it; Lord knows how they do it if diagnostics show it's low.
Further to davebro's post, my Volvo selector can be moved without the ignition on by using a hidden switch, as described in the handbook. Your Renault might be the same - check your handbook.
Tony - yes, I think it is something like that. I usually buy Haynes manuals for my cars but, for some reason, they never did one for mine; useful for finding out which jobs you can do and which are best left to a garage.
Probably have to take the gear selector to find out why the button won't depress or replace the gear selector ( doesn't have to be a brand new one, breakers will have one or Ebay probably ).
Regarding changing the fluid most modern auto boxes are pretty much sealed for life units these days, you have to half dismantle them to drain the fluid.