This occurs when I try to install CocoaPod as recommended in the React Native Docs referred here
https://reactnative.dev/docs/environment-setup#cocoapods
In my case I solve this by upgrading Ruby to 2.6.6
Warning: Please check the version again. This is as of 18 April 2021
This is done cleaner by Ruby version manager
Then do the upgrade
And it works !
Cheers !