You have to install the base of the machine first, it will take up room along that big threaded tube.
Install the boiler body into the base, THEN mount the heating element plate.
Edit: sorry, I didn't watch your video, I just looked at the picture without playing it. After watching your video and listening to it, its clear that either your boiler threads or the ring threads are partially stripped. The proper way to fix it would be to buy a new ring or new boiler, or both; but that is very expensive. If it does thread on partially like your video suggests, then you might try building the threads up slightly; but that's not an approved way of fixing it. Perhaps some very thin metallic tape wrapped over the threads? I've never tried that, though. Depending on the thickness of the metallic tape, it may not be possible to screw the ring back on... so it looks like at the very least you should try getting a new ring and seeing if it continues to slip with the new ring. It is cheaper than getting a new boiler
But, before spending money, try what I suggested above. Get 2 new gaskets, one above the base and 1 below it, and put the boiler into place with the ring... see if it continues to slip when you do that. The video sort of shows threads that are still in pretty good condition, so you might get lucky and have the bad threads only lower down like where it is in your video.