C# windows form SQL dateadd et like

I do not have the answer for you, my MS Access is a little rusty and Dates in there were not my strong point.
Examples of using dates as criteria in Access queries - Access[^]

However; I am obligated to point out that you are using code that is susceptible to SQL Injection. You should NEVER piece together an SQL statement from combining strings together.
The proper way to add the variables into this would be to use placeholders within your query and then add Parameters to the OLE Command. OLE restricts the placeholders to the question mark (?), and the values that are added need to be in the exact order to fill in the prepared statement.

This is roughly what your query should look like

connectionString = @"Provider=Microsoft.ACE.OLEDB.12.0;Data Source=|DataDirectory|\resources\BDD_Suivi_vente.accdb;Persist Security Info=False";

sql = "Select * from Tble_MES_bds_121 WHERE (Statut LIKE ? + '%') AND (Email LIKE ? + '%') AND (Date_MES IS NULL) AND (DATEadd(dd, -7, CONVERT(Date_fin_MSTPE)) Like ? '%')";

OleDbCommand cmd = new OleDbCommand(sql, connection);
cmd.Parameters.AddWithValue("@Statut", RechMstpe18);
cmd.Parameters.AddWithValue("@Email", RechEmail19);
cmd.Parameters.AddWithValue("@MSTPE", date1);

sql = "Select * from Tble_MES_121 WHERE (Statut LIKE '" + RechMstpe18 + "%')" + " AND (Email LIKE '" + RechEmail19 + "%') AND (Date_vente IS NULL)";

Not necessary a solution to your question, but another problem you have.
Never build an SQL query by concatenating strings. Sooner or later, you will do it with user inputs, and this opens door to a vulnerability named "SQL injection", it is dangerous for your database and error prone.
A single quote in a name and your program crash. If a user input a name like "Brian O'Conner" can crash your app, it is an SQL injection vulnerability, and the crash is the least of the problems, a malicious user input and it is promoted to SQL commands with all credentials.
SQL injection - Wikipedia[^]
SQL Injection[^]
SQL Injection Attacks by Example[^]
PHP: SQL Injection - Manual[^]
SQL Injection Prevention Cheat Sheet - OWASP[^]
How can I explain SQL injection without technical jargon? - Information Security Stack Exchange[^]

Hello,
thank you for your interest in my code.
Here is the excerpt of the modified move.

 string connectionString = null;
 OleDbConnection cnn;
 string sql = null;
 OleDbDataReader reader;

 string RechMstpe18 = "MSTPE";
 string RechEmail19 = "NON";

 string date1 = DateTime.Now.ToShortDateString();   // DAte J

//   MessageBox.Show("" + date1);

 connectionString = @"Provider=Microsoft.ACE.OLEDB.12.0;Data Source=|DataDirectory|\resources\BDD.accdb;Persist Security Info=False";
 sql = "Select * from Tble_MES_bds_121 WHERE " +
         "(Statut LIKE ? + '%')" + // Is there the word MSTPE
         " AND (Email LIKE ? + '%')" + // Is there the word NO for email
         " AND (Date_MES IS NULL)"; // The date DATE_MES is fast

     //  it's these three conditions are gathered so I can send a message by email ..
     //  BUT! I have to send an email 7 days before the end of the date(DATE_fin _MSTPE)

     cnn = new OleDbConnection(connectionString);
     cnn.Open();
     OleDbCommand cmd = new OleDbCommand(sql, cnn);

     cmd.Parameters.AddWithValue("@Statut", RechMstpe18);
     cmd.Parameters.AddWithValue("@Email", RechEmail19);
     cmd.Parameters.AddWithValue("@MSTPE", date1);

     reader = cmd.ExecuteReader();

     while (reader.Read())

     { //  HERE my code }

     reader.Close();
     cmd.Dispose();
     cnn.Close();

It is quite possible that several e-mails will leave at the same time.
For each line checked, when today's date is 7 days from the end date of (Date_fin_MSTPE).
Thank you