1. Loosen (turn left) the screw-nut of that slider;
2. Widen the exit slot slightly (you may use the screwdriver) until the slider can be pulled out.
Mounting instructions are in the User Manual and in “How to mount HEALTH GUARD” video on the ABOUT page.
Most likely, the coupler is wrongly inserted between the flange and the caster. Instead, insert it between the flange and the chair's base.
The longer - the better, there is no limit. Therefore do not rush, move slowly (~2 seconds each direction), and when one leg is tired - switch to another one.