Rails Console error after upgrading to Ubuntu 18.04

I had the same exact issue but we use rbenv instead of rvm. A similar answer to Brian's worked for me:

rbenv uninstall 2.4.1
rbenv install 2.4.1

Running the following code on the terminal solved the problem for me:

cd /lib/x86_64-linux-gnu
sudo ln -s libreadline.so.7.0 libreadline.so.6

You need to make sure readline is installed. You're using RVM, so you can run:

rvm requirements

and it should help make sure you have everything installed that you need, followed by:

rvm reinstall 2.4.1

if needed to make sure your ruby is good to go.