Error Database locked SQLite C#

Whenever I call the test() method the code runs until it reaches the Messagebox.show() When i try to insert into the "vandute" table i get the error "DATABASE IS LOCKED"

private void test()
    {
        int z;
        SQLiteConnection dbcon = new SQLiteConnection("Data Source=stefanauto.psa;Version=3;");
        dbcon.Open();
        using (SQLiteCommand sql = new SQLiteCommand(dbcon))
        {
            sql.CommandText = "select ID from clienti where CNP='" + cnp.Text + "'";

            z = Convert.ToInt32(sql.ExecuteScalar());


            MessageBox.Show(z.ToString());//The error begins after this line
            sql.CommandText = "insert into vandute(ID_MASINA,ID_CLIENT)values(1,2)";//Id_MASINA,ID-CLIENT ARE integers
            sql.ExecuteNonQuery();
        }
    }

Can you please explain me what am I doing wrong?