Error"must declare variable @studentnr".how can I solve it?

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C#

WPF

MySQL

MVVM

Hello,I have a problem in my program when I want to insert data into the db.This is my code:

public void SaveData(object param)
      {
          string connstring = null;
          connstring = "Data Source=(LocalDB)\\MSSQLLocalDB;AttachDbFilename=C:\\Users\\user0909\\Documents\\AttendanceStudents.mdf;Integrated Security=True;Connect Timeout=30";
          try
          {
              using (SqlConnection con = new SqlConnection(connstring))
              {


                  string sql = "Insert into RegisterStudent(SN,sNr,fName,lName,semester)Values(@sn,@studentNr,@fName,@lName,@semester)";
                  con.Open();
                  using (SqlCommand cmd = new SqlCommand(sql, con))
                  {
                      cmd.Parameters.AddWithValue("@SN", SqlDbType.Int).Value = sn;
                      cmd.Parameters.AddWithValue("@sNr", SqlDbType.Int).Value = studentNr;
                      cmd.Parameters.AddWithValue("@fName", SqlDbType.NVarChar).Value = fName;
                      cmd.Parameters.AddWithValue("@lName", SqlDbType.NVarChar).Value = lName;
                      cmd.Parameters.AddWithValue("@semester", SqlDbType.Int).Value = semester;
                      if(!string.IsNullOrEmpty(sql))
                      {
                          cmd.CommandText = sql;
                          cmd.ExecuteNonQuery();

                      }

                  }
              }
          }
          catch (Exception ex)
          {
              MessageBox.Show(ex.Message);
          }
      }

I insert all the values in the textboxes,but I get the error"

Must declare variable @studentNr"

This is my table:

CREATE TABLE [dbo].[RegisterStudent]
(
	[SN] INT NOT NULL PRIMARY KEY, 
    [sNr] INT NOT NULL, 
    [fName] NVARCHAR(MAX) NOT NULL, 
    [lName] NVARCHAR(MAX) NOT NULL, 
    [semester] INT NULL
)

What I have tried:

This is what I have tried:

SqlParameter StudentNR = cmd.Parameters.AddWithValue("@sNr", SqlDbType.Int);

             cmd.Parameters["@sNr"].Value = studentNr;
             if (StudentNR.Value == null)
             {
                 StudentNR.Value = DBNull.Value;
             }

I tried this approach,but I still get the error?What could be the problem?Thank you in advance!