so I also had the same problem. This is probably happening because your computer has an older version of ruby. So you need to first update your ruby. Mine worked for ruby 2.6.3 version.I got this solution from sStackOverflow,
You need to first open terminal and put this code
curl -L https://get.rvm.io | bash -s stable
Then put this command
rvm install ruby-2.6
This would install the ruby for you if it hasn' t been installed.After this just update the ruby to the new version
rvm use ruby-2.6.3
After this just make ruby 2.6.3 your default
rvm --default use 2.6.3
This would possibly fix your issue. You can now put the command
sudo gem install cocoapods
And the command
pod setup
I hope this was useful