Unable to add widgets to sidebars in custom theme

You are using get_sidebar incorrectly. That function “Includes the sidebar.php template file from your current theme’s directory.” And if you include a parameter, it loads a file matching that parameter name, not a sidebar of that name.

If a name ($name) is specified then a specialized sidebar
sidebar-{name}.php will be included.

What you want is dynamic_sidebar instead.

dynamic_sidebar('footer_widget_area');