JAVA cannot make a static reference to non-static field

You are calling instance methods and fields from within a static method, something that can't be done because instance fields and methods don't exist without an object, and inside of the main method there is not this object. You must instead create an instance of the class, and then call the methods on the instance.

public class Cerchio{

  float r;
  float area;
  float cfr;
  final double pi = 3.14;

  public static void main(String[] args){
    System.out.println("CIRCLE PROGRAM\n");

    Cerchio cerchio = new Cerchio();
    cerchio.r = 5;
    cerchio.c_cfr();
    cerchio.c_area();
    System.out.ptintln("The cir is: " + cerchio.cfr);
    System.out.println("The area is: " + cerchio.area);
  }

  float c_cfr(){
    cfr =(float)(2 * pi * r); //casting
    return cfr;
  }

  float c_area(){
    area = (float)(pi * (r*r));
    return area;
  }

}

Lots of other problems,...

• You're accessing class fields directly, something that shouldn't be done. Instead, the fields should be private and you should use getters/setters/contructor parameters to get, set and set the fields.
• Your code is unindented making it very hard to read and understand.

Please search this site as this same question has been asked and answered a gabizillion times, and most likely there's an answer out there that is much better than mine. If found, then this question should be closed as a duplicate.

Edit
You state:

I didn't understand "Instead, the fields should be private and you should use getters/setters/contructor parameters to get, set and set the fields." I should write private float c_cfr() ?

Your fields are:

float r;
float area;
float cfr;

This is really not a field but a constant: final double pi = 3.14;

and can be replaced / improved by simply using Math.PI.

Your fields should be changed to:

private float r;
private float area;
private float cfr;

and you should only access them via public getter and setter methods, and only if absolutely necessary.