You reference a Linux distribution, so you need to install the readline development libraries On Debian based platforms, like Ubuntu, you can run: sudo apt-get install libreadline-dev and that should install the correct headers in the correct places,. If you use a platform with yum, like SUSE, then the command should be: yum install readline-devel
the error seems to be thrown when searching for /usr/local/opt/readline/lib/libreadline.7.dylib. Have you tried to symlink that? So something like: cd /usr/local/opt/readline/lib ln -s libreadline.8.0.dylib libreadline.7.dylib Just tried that on macOS Mojave, ruby 2.5.3p105 and Rails 5.2.2 and worked.
if you have upgraded PostgreSQL with homebrew (brew update && brew upgrade), macOS (eg. from Catalina to BigSur) then simply uninstall the pg gem: gem uninstall pg bundle install and the path will be corrected for you. No need to uninstall the whole PostgreSQL cluster.
I was getting the exact same error, but the above answers didn’t work for me. I had to reinstall postgresql. brew reinstall postgresql