Which action causes a 422 error ? If this is the one in the last screenshot of KARAM_SIDHU , this appears to have nothing to do with installing or enabling extension. You are just trying to save invalid CCS and 422 is a validation error saying right on the page that it cannot parse the CSS. The syntax error is probably because you didn't prefix your comment with a comment delimiter.
If you are stuck and think it's because of a bad manipulation at some point, I would suggest installing a fresh forum, either on the same server or locally, just to test if you have the same issues or not. If all works well locally or on a different host I would abandon the current hosting as this is making Flarum a lot more difficult than it has to be.
