Question

I’m trying to use this code to check if the system already exists a field with this value

Dim adap As New MySqlDataAdapter
Dim sqlquery = "SELECT * FROM client WHERE code ='"+ TxtCode.Text +"'"
                Dim comand As New MySqlCommand()
                comand.Connection = con
                comand.CommandText = sqlquery
                adap.SelectCommand = comand
                Dim data As MySqlDataReader
                data = comando2.ExecuteReader()
                leitor.Read()
                If (data(3).ToString) = code Then
                    MsgBox("already exists", MsgBoxStyle.Information)
                    TxtCode.ResetText()
                    TxtCode.Focus()
                Else
                    Console.WriteLine(insert("INSERT INTO client (name, tel, code) VALUES ('" & name & "', '" & tel & "')"))
                    con.Close()
                End If

Answer 1

You called leitor.Read() but you did not call data.Read() in your code

Also, since your query is SELECT * FROM client WHERE code = '1234', there is no need to check for data(3) == code. If records exist, data.Read() would be true

If data.Read() Then
   MsgBox("already exists", MsgBoxStyle.Information)
   TxtCode.ResetText()
   TxtCode.Focus()
Else
   Console.WriteLine(insert("INSERT INTO client (name, tel, code) VALUES ('" & name & "', '" & tel & "')"))
   con.Close()
End If

It is also a good practice to use Parameters instead of concatenation

  Dim sqlquery = "SELECT * FROM client WHERE code = @code"
  ...

  command.Parameters("@code", code); //this is safer

and for your insert

 "INSERT INTO client (name, tel, code) VALUES (@name, @tel)"