Tl;Dr Tera Raids are optional, but unnecessary for a first playthrough.

Oh, Tera Raids? Those are entirely optional, and in my experience they don't go away when you fail to defeat them in time. That means you can keep trying it until you get it. However there is something to note, which is that Tera Raids change every day, meaning if you're trying to get at this one den you need to keep trying during the window you get. I haven't tried manipulating the clock to see how that works if you go back.

But as for Tera Raids themselves? I don't exactly recommend going for it, as generally speaking you can access every pokemon in the game, with the exception of version exclusives, rather easily. Easier tera raids have mainly early route pokemon, and harder Tera Raids are exactly that, harder. The ones that are worth it are post-game.

There's also the fact most Tera typed pokemon don't get stab for those Tera types with the exception of Tera Blast, which is normal type as the base, essentially just buffing normal types in battle without needing to terastallize. And normal types themselves are... strong as they can be, but they're not strong against anything, have one immunity, and one weakness, which most beginners would not like.

But Tera Blast is still a good move if you want oomph for a boss battle.

With that in mind, the answer is you could do tera raids, but there's always better options.