Why do I get "No such file to load"?

Question

I am working on installing Open Project and I ran bundle exec rails server and got:

root@bistro2:/home/openproject/openproject# bundle exec
/usr/lib/ruby/vendor_ruby/bundler/rubygems_ext.rb:8:in `require': no such file to load -- rubygems (LoadError)
        from /usr/lib/ruby/vendor_ruby/bundler/rubygems_ext.rb:8
        from /usr/lib/ruby/vendor_ruby/bundler.rb:11:in `require'
        from /usr/lib/ruby/vendor_ruby/bundler.rb:11
        from /usr/bin/bundle:4:in `require'
        from /usr/bin/bundle:4
root@bistro2:/home/openproject/openproject#