The most likely cause is that Tailwind 3.4.x has changes that affect how external CSS is processed. I've seen this before with other packages. Try these fixes:
Downgrade Tailwind to a slightly older version:
This version is more stable with external CSS libraries like Swiper. I had to do this on my last project too.
Or you could exclude Swiper CSS from Tailwind processing. You'll need to modify your build config to tell it not to run Swiper's CSS through the Tailwind processor.
The error is happening because Tailwind is trying to process Swiper's CSS files and can't find some property it expects (future). Either solution should fix it - I'd try the downgrade first since it's quicker.