The project relies on extensions that are installed separately to pull content from different services (mangadex, mangaplus, etc.). The other day, the tachiyomi devs removed almost all the extensions that they were hosting/maintaining after a threat of legal action from a Korean webtoon company. Here is a relevant discussion thread.

Here is a guide to a third party extensions repo that was linked in a discussion thread the other day.

I haven’t personally done this with my Tachiyomi yet, but have heard from others it has worked well.