That wiki page was outdated in required Ruby version, I've updated it to accurately say that you'll need Ruby >= 2.6. I've also updated the supported version of Ubuntu to 20.04+ as that's the one that comes out of the box with a supported Ruby version. If you're using an older Ubuntu version, you'll need to install a newer Ruby version using something like rvm or a ppa (e.g. brightbox).

Alternatively, you could use slate 2.12.0 which should work on Ruby 2.5, but will have outdated version of nokogiri dependency which does have a security vulnerability reported against it, but is for a feature that's not used for slate so it should be fine. The other patches from 2.13.0 onwards should apply fine as well against the older version.