Each lesson unlocks as you successfully complete the previous lesson.
You seem to have wiped out the template so I will fill that in a bit for you - it makes is easier for others to help.
Exercise URL: https://www.codecademy.com/en/courses/deploy-a-website/lessons/create-static-site/exercises/jekyll-install
Code that you typed (I will assume, but if not please correct me):
gem install jekyll
Error message that you are seeing: ?? this I don't know