Rails/Ruby Error When Creating Database: Unable to load the EventMachine C extension

try to re-install the gem 'eventmachine'

$ gem uninstall eventmachine

then

$ bundle install  ( in the same folder with your Gemfile) 

or just

$ gem install eventmachine ( if you are not using the Gemfile) 

see: https://stackoverflow.com/a/15140634/445908

Someone kind of answered in comments...see below.

Basically though I had seperate versions of Ruby, which was getting confusing because some gems were only installed on certain versions of ruby.

Fix was totally uninstalling ruby and rails, then reinstalling. Fixed it!

This helped:

This comment in another thread seems promising for your issue. Can you ensure that you have "gem eventmachine" or similar in your Gemfile, and can successfully bundle? – Scott Helm Jan 6 at 6:24

I got the same error on Windows after upgrading ruby from 1.9.3 to 2.4. I tried every fix I found but the only thing that helped in the end was (not that obvious and therefore hard to find) to reinstall the new Ruby again with 32 bit instead of 64.

Also I installed Eventmachine with

gem install eventmachine --platform=ruby

to force native compilation, since I always get problems with msgpack, that can be solved like this (haven't tried it with the default x86-mingw32)