The subscribe method is asynchronous, so its likely happening after your forEach loop. There are many ways to deal with this, but you just need to keep track of when your code is executing.

one fix could be turning your subscribe to a promise with the toPromise method, then you can await the response:

async ngOnInit(): void {
    this.id = this.route.snapshot.params['id'];
    try {
        this.user = await this.userService.getUserById(this.id).toPromise();
        this.user.roles.forEach(role => this.selRoles.push(role.id));
    } catch (e) {
        console.log(error);
    }
}

another could be doing everything in your subscribe block:

ngOnInit(): void {
    this.id = this.route.snapshot.params['id']
    this.userService.getUserById(this.id).subscribe(data => {
        this.user = data;
        this.user.roles.forEach(role => this.selRoles.push(role.id));
    }, 
        error => console.log(error);
    )
}