How to set an admin password with passport node - node.js

Question

This website is only going to have admin users, I have created the schema, the registration view and the route, however something is bad with the logic in the route.
/register page only have three inputs, username, password and email address. At the moment, if the user uses any random password to enter, is showing a flash error saying the password is not valid and redirecting me to the landing page, however, it is still creating the new user in the database.
Also, if the user uses the right password same thing as above is happening.
What am I doing wrong?
here is my code:
Schema
var UserSchema = new mongoose.Schema({
username: {
type: String,
unique: true,
required: true
},
password: {
type: String
},
email: {
type: String,
unique: true,
required: true,
lowercase: true,
},
resetPasswordToken: String,
resetPasswordExpires: Date,
// isAdmin: {type: Boolean, default: false, required:true}
});
Register view
<form class="form-group" action="/register" method="POST">
<div class="form-group">
<input class="form-control mb-1" type="text" name="username">
</div>
<div class="form-group">
<input class="form-control mb-3" type="password" name="password">
</div>
<div class="form-group">
<input class="form-control" type="email" name="email">
</div>
<button class="btn btn-outline-info w-100">Register!</button>
</form>
Route
// register route
router.get("/register", function(req, res) {
res.render("register", {page: "register"});
});
// handle sign up logic
router.post("/register", function(req, res){
var newUser = new User({
username: req.body.username,
email: req.body.email
});
User.register(newUser, req.body.password, function(err, user){
if(err){
console.log(err);
return res.render("register", {error: err.message});
} else {
if(req.body.password !== process.env.ADMIN_CODE) {
req.flash('error', 'the password is not valid');
res.redirect("/");
} else {
passport.authenticate("local")(req, res, function(){
req.flash("success", "Great! Welcome! " + req.body.username);
res.redirect("/");
});
}
}
});
});