When trying to raise the server with Rails from the terminal, it turns out that I have the following problem:
MacBook-Pro-de-Brian:~ Brian$ rails new twitter
/System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/lib/ruby/2.3.0/rubygems.rb:241:in 'bin_path': can't find gem railties (>= 0.a) (Gem::GemNotFoundException)
from /usr/local/bin/rails:22:in '<main>'