Can you tell me why I'm getting a "string cannot be converted to int" error

I have an assignment which asks for everything I have in the code below. That all works fine - I just need to calculate any monthly hours over 160 hours to be paid at 1.5 times the normal hourly rate. My math seems sound and calculates fine:

((hours - 160) * overtime) + (160 * hourlyRate)

But I dont know if I'm putting this if statement in the right method or if it even should be an if statement. My increase/decreasePay methods are working prior to this and they need to stay. I removed some things so it's easier to read.

HourlyWorker Class:

public class HourlyWorker extends Employee
{
private int hours;
private double hourlyRate;
private double monthlyPay;
private double overtime = (1.5 * hourlyRate);

public HourlyWorker(String last, String first, String ID, double rate)
{
   super(last, first, ID);
   hourlyRate = rate;
}

public void setHours(int hours)
{
   this.hours = hours;
}

public int getHours()
{
   return hours;
}

public void setHourlyRate(double rate)
{
   this.hourlyRate = rate;
}

public double getHourlyRate()
{
   return hourlyRate;
}


public double getMonthlyPay()
{
   if (hours > 160)
   {
      monthlyPay = ((hours - 160) * overtime) + (160 * hourlyRate);
   }
   else 
   {
      monthlyPay = hourlyRate * hours;
   }
   return monthlyPay;
}

public void increasePay(double percentage)
{
   hourlyRate *= 1 + percentage / 100;
}

public void decreasePay(double percentage)
{
   hourlyRate *= 1 - percentage / 100;
}

}

What I'm testing with:

public class TestEmployee2
{
   public static void main(String[] args)
   {
   Employee [] staff = new Employee[3];
      HourlyWorker hw1 = new HourlyWorker("Bee", "Busy", "BB1265", 10);
       
      hw1.setHours(200);    
      staff[0] = hw1;

   System.out.println(staff[0].getMonthlyPay());
   staff[0].increasePay(10);
   System.out.println(staff[0].getMonthlyPay());
}
}
Output is:
1600 (initial monthly rate, with 40 overtime hours and 160 regular hours)
1760 (10% increase to the monthlyPay)

Should be:
2006
22

06.6

As @NomadMaker mentioned, the problem is in your addArtist method.
Your current method:

   public void addArtist(String artistName, String genre)
   {
      this.artists.add(artist, genre); 
   }

Remember that this.artists is a list which can store Objects of type Artist.
Therefore you should create a new artist with the new parameters. Something like:

public void addArtist(String artist, String genre)
   {
      this.artists.add(new Artist(artist, genre)); 
   }

As you might be guessing, you do not have a constructor of Artist with two parameters (should accept name, and genre). Therefore, you should add this constructor to your code:

public Artist(String name, String genre) {
 this.name = name;
 this.genre = genre;
}

Error explanation:

artists is a list, what you are doing when calling this.artist.add(artist, genre) is calling a method which belongs to the list collection that has this signature: add(int index, Artist artist) the index will be the index to place the artist (if the order matters for you).