It’s because you are running out of memory. Try adding more swap: www.digitalocean.com/community/articles/how-to-add-swap-on-ubuntu-12-04. Or bump up to a bigger droplet. Ref: www.digitalocean.com/community/questions/bundle-install-gets-killed
How To Add Swap Space on Ubuntu 20.04
www.digitalocean.com/community/tutorials/how-to-add-swap-space-on-ubuntu-20-04