Pretty permalink not working with categories in custom theme

I’ve written a custom theme that doesn’t work with pretty permalinks properly. The funny thing is that I can link to posts and pages, but I can’t link to categories. Everything works except for categories. When the link points to any category, it returns “Error 404 Not Found”.

My website is https://www.gevin-ent.com Could anyone please help me? This has been puzzling me for a long time and I’ve tried everything (turned off all plugins, reset htaccess, etc. etc.). Only default permalink works with everything.