error CS1520: Class, struct, or interface method must have a return type

Error:

Assets/Scripts/Character Classes/ModifiedStat.cs(7,16): error CS1520: Class, struct, or interface method must have a return type

Script:

using System.Collections.Generic;

public class ModifiedStabt : BaseStat {
	private List<ModifyingAttribute> _mods;
	private int _modValue;
	
	public ModifiedStat() {
		_mods = new List<ModifyingAttribute>();
		_modValue = 0;
	}
	
	public void AddModifier( ModifyingAttribute mod) {
		_mods.Add(mod);
	}
	
	private void CalculateModValue() {
		_modValue = 0;
		
		if(_mods.Count > 0)
			foreach(ModifyingAttribute att in _mods)
				_modValue += (int)(att.attribute.AdjustedBaseValue * att.ratio);
	}
	
	public new int AdjustBaseValue {
		get{ return BaseValue + BuffValue + _modValue; }
	}
	
	public void Update() {
		CalculateModValue();
	}
}
	
public struct ModifyingAttribute {
	public Attribute attribute;
	public float ratio;
}

You class name is called ModifiedStabt (with a b before the last t), while your constructor is called ModifiedStat (without the b).

Use an editor with error highlighting like Visual Studio, and you’ll see these errors easier.

Error CS1520: Class, struct, or interface method must have a return type (CS1520)
using System;
using gtk;
using Pango;
using System.Threading;
using System.IO.Ports;

public partial class MainWindow: gtk.Window
{
int minutos, segundos, bandera = 0;
Thread reloj;
SerialPort serial;

public MainV ():base (Gtk.WindowType.Toplevel)
{
	Build ();
	cajatextoA.ModifyFont(FontDescription.FromString("Arial 25"));
	cajatextoB.ModifyFont(FontDescription.FromString("Arial 25"));
	serial = new SerialPort();//Instanciamiento del puerto serial
	serial.PortName = "/dev/ttyACM0";//Se indica cual puerto será utilizado
	serial.BaudRate = 9600;//La velocidad del puerto
	serial.Open();
}

protected void OnDeleteEvent (object sender, DeleteEventArgs a)
{
	Application.Quit ();
	a.RetVal = true;
	if(serial.IsOpen)//Si el puerto aun esta abierto...
		serial.Close();
}
protected void OnButton1Clicked (object sender, EventArgs e)
{
	minutos = Convert.ToInt32(cajatextoA.Text)+10;
	cajatextoA.Text = ""+minutos;
	serial.Write("0");
}	

protected void OnButton2Clicked (object sender, EventArgs e)
{
	minutos = Convert.ToInt32(cajatextoA.Text)+1;
	cajatextoA.Text = ""+minutos;
	serial.Write("0");
}	

protected void OnButton3Clicked (object sender, EventArgs e)
{
	segundos = Convert.ToInt32(cajatextoB.Text)+10;
	if(segundos>59){
		segundos = segundos - 60;
		minutos = Convert.ToInt32(cajatextoA.Text)+1;
		cajatextoA.Text = ""+minutos;
		segundos = 0;
		cajatextoB.Text = ""+segundos;
	}else
		cajatextoB.Text = ""+segundos;
	serial.Write("0");
}	

protected void OnButton4Clicked (object sender, EventArgs e)
{
	segundos = Convert.ToInt32(cajatextoB.Text)+1;
	if(segundos>59){
		segundos = segundos - 60;
		minutos = Convert.ToInt32(cajatexto.Text)+1;
		cajatextoA.Text = ""+minutos;
		segundos = 0;
		cajatextoB.Text = ""+segundos;
	}else
		cajatextoB.Text = ""+segundos;
	serial.Write("0");
}	

protected void OnButton5Clicked (object sender, EventArgs e)
{
	serial.Write("2");
	if(bandera == 1){
		reloj.Resume();
		bandera = 0;
	}else{
		segundos = Convert.ToInt32 (cajatextoB.Text);
		minutos = Convert.ToInt32 (cajatextoA.Text);
		reloj = new Thread(cuentaAtras);
		reloj.Name = "Cuenta hacia atras";
		reloj.Start();
		reloj.Priority = ThreadPriority.Highest;
	}
	serial.Write("0");
}

protected void OnButton6Clicked (object sender, EventArgs e)
{
	serial.Write("3");
	if(bandera == 0){	
		reloj.Suspend();
		bandera++;
	}else{
		reloj.Abort();
		cajatextoA.Text = "0";
		cajatextoB.Text = "0";
		bandera = 0;
	}
	serial.Write("0");
}

public void cuentaAtras ()
{
	while (segundos != 0 || minutos != 0) {
		if (segundos > 0) {
			segundos--;
			cajatextoB.Text = Convert.ToString(segundos);
		} else if (minutos > 0) {
			minutos--;
			segundos = 59;
			cajatextoA.Text = Convert.ToString(minutos);
			cajatextoB.Text = Convert.ToString(segundos);
		}else{
			;
		}
		Thread.Sleep (TimeSpan.FromSeconds (1));
	}
	serial.Write("1");
	reloj.Abort ();
}

}