I have justs installed the latest version of Krita, but I am unable to open .tif files, I get an error saying the files cannot be parsed.

If you're on macOS, that's because of a build problem in 4.2.8. Please check the 4.2.9 beta which will be released on Monday. You can download it already: https://download.kde.org/unstable/krita/4.2.9-beta1/