I've got another issue here. Operator "+" cannot be applied to `java.lang.String', `void`

I have no clue how to fix this error at all I've been following every step of a video for Returning a value from a class method and I get "Operator "+" cannot be applied to 'java.lang.String', 'void'. Where I put the "------>" and "<--------" in between those are the two lines which have the issue.

package com.company;

public class Main1 {

     public static void main(String[] args) {
         aircraft EurofighterTyphoon = new aircraft();
         aircraft F15_Fighter = new aircraft();

         EurofighterTyphoon.passengers = 4;     // The amount of passengers for the Eurofighter Typhoon
         EurofighterTyphoon.cruiseSpeed = 140;   // The cruise speed of the Eurofighter Typhoon
         EurofighterTyphoon.fuelCapacity = 56.5; // The fuel capacity of the Eurofighter Typhoon
         EurofighterTyphoon.fuelBurnRate = 9.5; // The fuel burn rate of the Eurofighter Typhoon

         F15_Fighter.passengers = 4; // The amount of passengers for the F15 Fighter
         F15_Fighter.cruiseSpeed = 140; // The cruise speed of the F15 Fighter
         F15_Fighter.fuelCapacity = 56.5; // The fuel capacity of the F15 Fighter
         F15_Fighter.fuelBurnRate = 9.5; // The fuel burn rate of the F15 Fighter

         ----> System.out.println("Eurofighter Typhoon Endurance is: " + 
         EurofighterTyphoon.calculateEndurance());

         System.out.println("F15 Fighter Endurance is: " + F15_Fighter.calculateEndurance()); <--------
     }
  }
class aircraft1 {
    int passengers;           // Number of people
    int cruiseSpeed;          // miles per hour
    double fuelCapacity;      // gallons
    double fuelBurnRate;      // Gallons per hour

    // Method to calculate aircraft endurance

    double calculateEndurance() {
        double endurance;

        endurance = fuelCapacity / fuelBurnRate;

        return endurance;

    }
}