Should I learn next js after react? What you guys think? Gimme some advice.

Well yes sure there is no harm in learning it if you want to. But are you going to stick with frontend development or move into backend as well if you don't know it? Because if so then I would choose to learn Node.js next so that you become full-stack. Then you can learn Next.js if you want to. It's essentially React anyway so not that hard to learn.

Yes, you should have basic understanding about the React as Next.js is just React Framework for Server Side Rendering.

Please hack the React crash course for initial knowledge of React :)