Installing ruby with RVM cannot find <CoreFoundation/CFString.h>

Apologies for asking an uninteresting question, I am currently trying to
learn ruby for a senior capstone project by creating a RESTFUL api with it.
I have been trying to install ruby using rvm for a week now and I have had
little success. Both rvm and rbenv complain about not being able to find
CoreFoundation/CFString.h on OSX. I know this is more of a question related
specifically to my machine, but how can I fix my machine? I have posted on
Apple forums, stackoverflow 3-4 times, etc., not sure where else to go from

I am on OSX 10.14.4 with XCode 2.2 installed. More detailed information can
be viewed in the forum posts linked below.

I have homebrew and I am worried about having two versions of ruby
conflicting with it.

Here is my post on apple's forums:

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I'm having issues right now with multiple ruby versions being installed
(the osx one and the homebrew one I think). For example with rails right
now trying to generate a new model I get:

`warn_for_outdated_bundler_version': *You must use Bundler 2 or greater
with this lockfile. (**Bundler::LockfileError**)*

Yet I have bundler 2.0 installed. Are there any grand master ruby wizards
out there that can give me some guidance.

Basically I just want to ignore the OSX version of ruby (and leave it
unchanged), and manage one using rvm where I can change the version based
on what tutorial I am using (or even just maybe let it stay at the newest
up to date one).

If I type ruby -v I get 2.6.2
But if I generate a new project with rails for an api, the gemfile
automatically goes to 2.3.7 and I have problems.

Thank you,

Nicholas Gallimore