Macos – “You don’t have write permissions for the /usr/bin directory.” when installing Sass using the Gem command

According the question I see a few steps in order to allow writing to /var/lib/gems:

1. Create gems (if not any) group, and add yourself to the group:

   $ groupadd -f gems
   $ usermod -aG gems $(whoami)
   

2. When reenter to the session:

   $ sudo -u $(whoami) bash
   

3. Allow write for a gems group to /var/lib/gems/, and set the setgid bits:

   $ chown :gems /var/lib/gems/
   $ chmod g+sw /var/lib/gems/
   

4. Try installing gems into /var/lib/gems/.

NOTE: I strongly advice you to control rubies and gems via rbenv/rvm keeping them in a local home folder. Please refer to the answer to know how to properly setup your project.

by default, in a Forms app, you're storing the MainPage which should be a NavigationPage in this case.

public class App : Application
{
    public App ()
    {
        // The root page of your application
        MainPage = new NavigationPage {

So you can use it to get to the Navigation object.

protected override async void OnResume ()
{
    var nav = MainPage.Navigation;

    // you may want to clear the stack (history)
    await nav.PopToRootAsync (true);

    // then open the needed page (I'm guessing a login page)
    await nav.PushAsync(new LoginPage());
}