Change your code...
ScriptManager.RegisterStartupScript(this, this.GetType(), "errormsg", "alert('CLUSTER FROM A SINGLE GP AT A TIME !!!!')", true);
to...
ScriptManager.RegisterStartupScript(this, this.GetType(), "errormsg", "alert('CLUSTER FROM A SINGLE GP AT A TIME !!!!');", true);
Just a semicolon and you are good.
This is because the code which will follow this line when rendered won't work because this line is invalid due to the missing semicolon.