ERROR: Error installing rails: activesupport requires Ruby version >= 1.9.3.

Check whether you are trying to install the correct version of Rails that are compatible with your Ruby Version.

You can check the ruby version by running the cmd "ruby -v"

Rails Version	Possible Ruby Versions	Recommended Ruby Version
1.0–2.1	1.8.6	1.8.6
2.2	1.8.6 or 1.8.7	1.8.7
2.3	1.8.6, 1.8.7, or 1.9.1	1.8.7
3.0–3.2	1.8.7, 1.9.2, or 1.9.3	1.9.3
4.0–…	1.9.3, 2.0.x	2.0.x

then try to install the rails

sudo gem install rails --version 3.2.12