Is it compulsory to know ES6 in order to be good at web design? Cos I've been stuck at ES6 for a while especially the Promise

Answer 1

It depend of what kind of project you will make, some dont need things like promise.

But I suggest you to learn it, take your time, practice, read from many resources.
I strugle before to figure out
filter, map - array methods
Arrow functions was too complicated for me, but now I only use this 😄

Check few resourse from internet, make sure you follow example and code. Sometimes you just need to read same topic explained in diferent words to understand it.