I was going to buy a theme but I read that it is compatible only with wordpress 5.7.x versions and lower. I’m using Wordpress 5.8 version.
If the theme compatible with 5.7.x then I think it will work fine with 5.8.x. But you can ask it directly to the theme author by posting comments going through the theme Comments page. The theme author will answer your query.
You certainly do not want to use a theme which does not support latest version of WordPress. It may be fine now, but you do not want to be stuck on the outdated version of WP longterm.
That said, if by “I read that it is compatible only with WordPress 5.7.x” you meant that you’ve just read that on the item page, that doesn’t mean the theme won’t work with the WP 5.8. This info has to be updated manually by the author, so maybe he just forgot to change it and the theme may work well with WP 5.8.
Anyway, get in touch with the author and ask him about the compatibility. I mean at least you will see how long it takes him to reply, in case you will need his assistance after the purchase.