I have had this problem as well. It looks like the Rubygem needs to be updated past version 3.0.0
in order for footnotes to work. There are changes in the master
branch which aren't available on Rubygems.
To fix your issue for the time being, source the gem from the Redcarpet Github repo:
gem 'redcarpet', github: 'vmg/redcarpet', branch: 'master'
Then run bundle install
. Footnotes should now work.
I've opened an issue on the Redcarpet repo to try to get someone to update the Rubygem with the latest changes. I'll post back here if/when that's done.