Short: If the tailgate won't open when you press the button it's because the car thinks the roof is in the wrong position and it probably is. It is probably a faulty roof motor, gear or cables. (It could be faulty micro switches or release motors but both have been more reliable IME. I did have the ABS unit fail and that stopped the roof from working.) Roof reset seems to be press up on the roof switch for 30secs - if you hear a machine gun in the back read on.
Long story, take a seat: As of May 2021 my roof started to play up, clacking sounds and not wanting to open fully and then close fully. I sprayed the tracks with silicone (it had helped the door mirrors open smoothly), so surely that would help?! Well it did, but a month later and the tailgate wouldn't open. I got into the boot through the back seat, lay on my back looking up, removed the parcel shelf and then 4 Torx20 screws holding plastic trim and got fingers in to pull on the tailgate unlock actuators and the lid popped up. At this point the boot release button worked. So I removed the plastic trim 2xT20 under the R of Citroen to access the tailgate switch and tested it with a multimeter - all good with the switch, but not with the car.
I thought the roof might have a reset programme. The roof was closed, but perhaps not perfectly. With the roof switch in the closed position I pressed it up and there was a BRRRRR/RAT-TATATTAT/machine gun sound from the rear - Oh 5#!t, I thought (said).
So, it either needs a new gear (motor unit) or replacement cables.
Meanwhile, I have used kite string to pull on the actuators manually.
Hope this helps fellow owners.
Robi4997 London 21st June 2021
I have since found very detailed instructions in Italian http://www.citroen-club.it/forum/viewtopic.php?f=48&t=20272
I think it is worth doing parce que c'est le tout raison d'être pour ma voiture n'est pas?
robi3997
June 2021