Nodemailer Oauth2 error : unauthorized_client

I'am trying to send mail using Oauth and nodemailer on a nodejs app, I did it without Oauth but my password was wrote in the code so I turn myself to Oauth.

I only want to connect myself to send mail in an automatic way. I have settup a project and a service account on google cloud platform. I added the gmail api and wrote some code :

 var smtpTransport = nodemailer.createTransport({
   host:'smtp.gmail.com',
   port:465,
   secure:true,
   auth:{
     type: 'OAuth2',
     user: 'thomas.legrand.test@gmail.com',
     serviceClient:config.client_id,
     privateKey:config.private_key
   }
 });
 var mail = {
   from: "thomas.legrand.test@gmail.com",
   to: "thomas.legrand26@gmail.com",
   subject:"Un sujet abstrait",
   html:"<h1> Ceci est un mail de test </h1><h2> et ceci un sous titre </h2> "
 };

 smtpTransport.on('token', token => {
    console.log('A new access token was generated');
    console.log('User: %s', token.user);
    console.log('Access Token: %s', token.accessToken);
    console.log('Expires: %s', new Date(token.expires));
});

smtpTransport.sendMail(mail, function(error, response) {
        if(error) {
          console.log("Erreur lors de l'envoie du mail ");
          console.log(error);
        }else {
          console.log("succes")
        }
        smtpTransport.close();
      });

But I get an error (unauthorized_client) which I can't solve.

I hope you can help me or give me hints at least !