Jekyl requires ruby v2.4+
ruby -v
Update ruby to 2.4+ (if not currently). There's a rbenv version file .ruby-version in the root which should set your ruby version to 2.6.3
https://github.com/rbenv/rbenv#command-reference
Make sure ruby gems for 2.6 are in your PATH
export PATH=$HOME/.gem/ruby/2.6.0/bin:$PATH
jekyll serve