Updated code

import javax.swing.JOptionPane; //imports the joptionpane

public class Testing {
public static void main (String [] args) {

   int bonus = 100;  //assigns the bonus to be 100 ($100)


   String employee = JOptionPane.showInputDialog("Employee name:"); //runs a dialog box to collect string data for the employee's name
   int years = Integer.parseInt(JOptionPane.showInputDialog(employee + "'s years of service:")); //runs a dialog box to collect integer data for the years of service
   String rating = JOptionPane.showInputDialog(employee + "'s Rating:"); //runs a dialog box to collect string data for the employees performance rating

    if (rating.equalsIgnoreCase("Below average")) //if the rating string variable is equal to Below average, run this block of code.
      {
      JOptionPane.showMessageDialog(null, employee +  "is NOT eligble for a bonus on account of their " + rating + " performance despite their " + years + " years of service."); //if the rating entered is equal to below average message box will provided stating that the employee is ineligble
      return;
      }
    else if (rating.equalsIgnoreCase("Average"))
    {
    }
    else if (rating.equalsIgnoreCase("Above average"))               //proceed if only one of the options for text entry have been inputted
    {
    }
    else if (rating.equalsIgnoreCase("Outstanding"))
    {
    }
    else
    {
    JOptionPane.showMessageDialog(null, "ERROR - Please start again. Please pick between below average, average, above average and outstanding."); //error message to appear if any string entered into the dialog box doesnt match one of the above options
    return;
    }

   if (years < 1)  //run this block of code if the years are less than 1
   {
     JOptionPane.showMessageDialog(null, "This is not a valid input."); //provides error message if the years are less than 0
     return;
   }
     else if (years < 2) //run this code if the years of service are less than 2
     {
     JOptionPane.showMessageDialog(null, employee + " is NOT eligible for a bonus on account of their " + years + " years of service."); //provides a message detailing ineligbility when the user has less than 2 years of service
   return;
   }



   if (years >= 20) //run this block of code if the years of service are equal to or greater than 2 years
   {
    int final_bonus = (years * bonus); //calculating the bonus by multiplying the years by the bonus variable assigned at the top of the code
    JOptionPane.showMessageDialog(null, "Success! " + employee + " is eligible for a bonus of $" + final_bonus * 2 + " on account of their " + years + " years of service and their " + rating + " performance rating."); //provides a dialog box combining strings and variables to provide a complete bonus total
   }
      else if (years >= 2) //run this block of code if the years of service are equal to or greater than 20 years
   {
     int final_bonus = (bonus * years); //calculating the bonus by multiplying the bonus and the years together and then multiplying that by 2
     JOptionPane.showMessageDialog(null, "Success! " +  employee + " is eligible for a bonus of $" + final_bonus + " on account of their " + years + " years of service and their " + rating + " performance rating."); //provides a dialog box combining strings and variables to provide a complete bonus total
     return;
   }

  }
}