Force User Login

Log into your forum as an Admin
Open the Admin Control Panel, you can do this via the gears icon or by appending "/admin" to your forum URL
Open the "Appearance > Custom Content (HTML/JS/CSS)" page
Select the Custom Javascript tab

Paste in the following code:

$(document).ready(function () {
    function redirectAnon () {
        if (!app.user.uid && document.location.pathname !== '/login') {
            document.location = config.relative_path + '/login';
        }
    }
    
    $(window).on('action:ajaxify.end', redirectAnon);
    redirectAnon();
});

Click the Diskette icon to the bottom right of the page to save
Select the Dashboard menu
Restart your NodeBB instance