You are probably trying to create nested directories. Assuming foo does not exist, you will receive no such file or directory error for:
Dir.mkdir 'foo/bar'
# => Errno::ENOENT: No such file or directory - 'foo/bar'
To create nested directories at once, FileUtils is needed:
require 'fileutils'
FileUtils.mkdir_p 'foo/bar'
# => ["foo/bar"]
Edit2: you do not have to use FileUtils, you may do system call (update from @mu is too short comment):
> system 'mkdir', '-p', 'foo/bar' # worse version: system 'mkdir -p "foo/bar"'
=> true
But that seems (at least to me) as worse approach as you are using external ‘tool’ which may be unavailable on some systems (although I can hardly imagine system without mkdir, but who knows).