get_stylesheet_directory_uri() returns an URL. The requested file is called like any other publicly available resource: without the context of the include statement. This is like opening the PHP file in a browser directly. There is no WordPress context, and the functions add_action() or add_filter() are not defined.
Use get_template_directory() in parent themes and get_stylesheet_directory() in child themes.
Disable allow_url_fopen and allow_url_include in your development environment to catch errors like this early and with better error messages.
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