Error writing new record to the Access database

  

Tip : It may be important for you to start working with queries parameters. In the current way, it's very easy for an attacker to get SQL injection (read about it, here and here ) in your code.

You need to enter the date in the format YYYY-MM-DD HH: MM: SS and circled by hash / sharp or # ) or single quotes

That is, insert should look like this

INSERT INTO Cliente (descCliente, nomefantasia, endereco, complemento, bairro, cep, estado, 
                  cidade, telefone1, telefone2, celular1, email, CPF, RG, OEmissor, DtNasc)
VALUES ('fernando', 'fer', 'rua 1', '', 'limoeiro', '00125-455', 2, 1960,
        '(11) 1111-1111', '( ) -', '(11) 91111-1111', '[email protected]', ' . . -', 
        '111.111.111-11', 'SSP-PS', #1986-02-10 00:00:00#); 

Or (just the end so you do not keep repeating everything)

'1986-02-10 00:00:00'

To format the date, just use .ToString() .

Convert.ToDateTime(mskDOB.Text).ToString("yyyy-MM-dd HH:mm:ss");

I advise you not to use this way ... You are concatenating the values within the query. Try to impute the values using parameters. This will avoid problems that may come in the future SQL injection and will not give you these problems with date. The code looks like this:

string sql = "INSERT INTO Cliente (descCliente, nomefantasia, endereco, 
complemento, bairro, cep, estado, cidade, "
                + "telefone1, telefone2, celular1, email, CPF, RG, OEmissor, 
DtNasc)"
            //, CPF, RG, OEmissor, bloqueado, mensalista) "
            + "VALUES (@descCliente, @nomefantasia, @endereco, @complemento, @bairro, @cep, @estado, @cidade, "
            + "@telefone1, @telefone2, @celular1, @email, @CPF, @RG, @OEmissor, @DtNasc) ";
        //+ false + ", "
        //+ false + ");";
        OleDbConnection conn = new OleDbConnection(connStr);
        OleDbCommand cmd = new OleDbCommand(sql, conn);
        cmd.CommandType = CommandType.Text;

        cmd.Parameters.AddRange(new OleDbParameter[]
           {
               new OleDbParameter("@descCliente",  txtnome.Text),
               new OleDbParameter("@nomefantasia", txtFantasia.Text),
               ...
           });

        conn.Open();
        try
        {
            int i = cmd.ExecuteNonQuery();
            if (i > 0)
            {
                MessageBox.Show("inclusão efetuada com sucesso");
            }
        }
        catch (Exception ex)
        {
            MessageBox.Show(ex.Message);
        }
        finally
        {
            conn.Close();
        }