Why don't tooltips work in IntelliJ IDEA?

Question

IntelliJ IDEA

I downloaded IntelliJ IDEA for the first time, installed the JavaRush plugin. Previously, for example, when I wrote some class (for example, Integer) and put an end to it, hints with methods implemented in the class came up, there was highlighting, but now when I reinstalled and started a bare project without a plugin, there is nothing, even code highlighting. What to do? Do I need to install some plugins?

Answer 1

But is there such an inscription on top: "Project SDK is not Defined" ?