I had the same errors on macOS Catalina.
First make sure that the .bash_profile in your home directory includes this path:
(Show hidden files in Finder on Catalina: cmd + shift + fn + period)
export PATH="$HOME/.gem/ruby/2.6.0/bin:$PATH"
With macOS Catalina, Apple is now using Zsh as the default shell.
I solved the problem with switching the default shell to Bash.
Run this command in Terminal:
chsh -s /bin/bash