Everything broke! Can't deploy updates to my apps anymore.

$ fastlane -v
-bash: /usr/local/bin/fastlane: /System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/bin/ruby: bad interpreter: No such file or directory

$ ruby -v
ruby 2.6.3p62 (2019-04-16 revision 67580) [universal.x86_64-darwin19]

$ sudo gem install fastlane -NV
...
bunch of downloads
...
ERROR:  While executing gem ... (Gem::FilePermissionError)
    You don't have write permissions for the /usr/bin directory.

I don't know what -NV does. Just saw it on some post.

It turns out the reason why I don't have permission to write into /usr/bin is that no one can, even as root. The fix is to install into /usr/local:

$ sudo gem install fastlane -n /usr/local/bin

Phew, it works again!

$ fastlane -v                                
fastlane installation at path:
/Library/Ruby/Gems/2.6.0/gems/fastlane-2.133.0/bin/fastlane
-----------------------------
[✔] 🚀 
fastlane 2.133.0