Some people are having this problem if they have upgraded from rails 3.x to 4 and they still have a group :assets do
block in your gemfile. In rails 4 you have to get rid of the assets group
delete "group :assets do" and "end" from your gemfile, if they exist.
group :assets do
gem 'bootstrap-sass'
end
Other people with this error were able to fix it by simply restarting the server, still others were able to fix the same error with a small change to the lib/bootstrap-sass.rb file as documented here in a pull request.