I've recently installed the plugin "spring boot assistant" into my Intellij. When I was trying to run an old java file that I've completed before, a lot of the symbols were displayed as "cannot resolve symbol".
This is what my file looks like.

I think this might have something to do with maven, but I dont know how to fix it since there's no errors with running the program before installing spring-boot and maven. I've just started learning spring boot so I'm still unfamiliar with the functions. Please help me.
