passport google auth: connect-flash not working - node.js

Question

I'm trying to flash a message when someone try to login with gmail but gmail is not found in database.... everything works fine but if there is no gmail account in database then it should return a flash message but flash not work! I've searched a lot on this topic but everyone post about passport-local...
here is my code:
--middlewares
app.use(session({
resave: false,
saveUninitialized: true,
secret: "a87ds34cN5070AS%ASasyasaad"
}));
app.use(flash());
app.use(passport.initialize());
app.use(passport.session());
--passport codes
passport.serializeUser(function(user, done) {
done(null, user);
});
passport.deserializeUser(function(user, done) {
done(null, user);
});
passport.use(new GoogleStrategy({
clientID: process.env.GOOGLE_CLIENT_ID,
clientSecret: process.env.GOOGLE_CLIENT_SECRET,
callbackURL: process.env.GOOGLE_CALLBACK_URI
},
function(req, accessToken, refreshToken, profile, cb){
// check if this email exist on database
db.get("SELECT * FROM users WHERE gmail=?", [profile._json.email], (error, user) => {
if(error){
console.log("there is an error!");
return cb(error, false);
}
if(user){
/* if the email is exist but user fullname field is empty then retrive these data and update this user */
if(profile.displayName != user.full_name){
db.run("UPDATE users SET full_name=? WHERE gmail=?", [profile.displayName, user.gmail], (error, done) => {
if(error){
console.log("fname error: "+error);
}
});
}
/* if the email is exist but profile avatar is empty then retrive profile avatar url from gmail and update this user */
if(profile._json.picture.split("=")[0] != user.picture){
const profile_pic = profile._json.picture.split("=")[0];
db.run("UPDATE users SET picture=? WHERE gmail=?", [profile_pic, user.gmail], (error, done) => {
if(error){
console.log("picture error: "+error);
}
});
}
return cb(error, user);
}else{
// if the email isn't exist then show this flash msg
return cb(null, false, req.flash("flash_msg", "You don't have any permission to access this area!"));
}
});
}
));
--login & callback route
app.get("/login", (req, res) => {
res.render("login.html");
});
app.get("/auth/google", passport.authenticate('google', {scope: ['profile', 'email']}));
app.get("/auth/google/callback",
passport.authenticate('google', {
successRedirect: "/",
failureRedirect: '/login',
successFlash: true,
failureFlash: true
}),
function(req, res) {
res.redirect("/");
});