IntelliJ community edition Code COmpletion not working in imported Java projects

So, I've had this issue for a while. First with MOOC Java course when I was doing exercise I never got any Java code suggestions nor highlighted errors. I always had to wait to compile and find out about mistakes but I kinda ignored it.

Now, I'm doing another online course and when I'm doing a project from the scratch the code completion/suggestions work perfectly. But when the the project is downloaded and imported or cloned from Github there's not a single suggestion coming up or not even the error highlighting feature works.Not even the navigation tools like Go To -> implementation (Ctrl + b ) is finding any usages when it's clearly there.

Any idea on how to fix this?.