Cannot run scala in IntelliJ

Question

I'm struggling with running Scala Hello World project using IntelliJ IDEA Scala plugin. The project is created without any problems but when I create function which should be run, the run button doesn't appear. I've got no idea why it doesn't work.

Version of IntelliJ - 2020.3.1 (Community Edition) Version of Scala plugin 2020.3.18

1. I've created Scala/sbt project (I'd also tried Scala/IDEA)
2. Vanilla options (but I'd also tried other versions of JDK, Scala and sbt)
3. The run button is missing

My code is:

class Hello extends App {
  println("Hello world")
}

I've tried creating Scala worksheet and It works.

Answer 1

When you extend App, it needs to be as object Main extends App, not class Main extends App.

See Scala 2.13 specification 9.5 (emphasis mine):

A program is a top-level object that has a member method main of type (Array[String])Unit

The main method of a program can be directly defined in the object, or it can be inherited. The scala library defines a special class scala.App whose body acts as a main method

Answer 2

As Suma mentioned in his answer, you should use object, and not class. The code of the Hello file should be:

object Hello {
  def main(args: Array[String]): Unit = {
    println("Hello world")
  }
}

As stated in YOUR FIRST LINES OF SCALA. Please note that if you prefer to extend App, all of the statements will be executed, as stated in the last link:

The argument of the scala command has to be a top-level object. If that object extends trait scala.App, then all statements contained in that object will be executed; otherwise you have to add a method main which will act as the entry point of your program.

Which means you have another option:

object Hello extends App {
  println("Hello world")
}

I have created a gif that shows an example.