Ignoring ffi-1.13.1 because its extensions are not built. Try: gem pristine ffi --version 1.13.1

Basically, you need to do what the warning message says: install the Gem "ffi" in the specified version by running gem pristine ffi --version 1.13.1. However, usually you don't have write permissions on the system to install a Gem into the stated directory.

You could probably just use sudo, i.e. run the command sudo gem pristine ffi --version 1.13.1 and enter the password, but I guess this is generally not recommended as it is a security risk.

I installed it with like this:

gem install ffi --version 1.13.1 --user-install

Then I had to add the installation path to my PATH variable; in my case I added this line to my ~/.zshrc:

export PATH=$HOME/.gem/ruby/2.6.0/bin:$PATH

I'm really not an expert on Ruby stuff, but this solved the issue for me.