Why am I seeing "An error occurred while installing ruby" on the heroku-16 stack?
heroku-16 is the new default stack on Heroku. As part of the transition older, unsupported Ruby versions were not ported over to the new stack which means that anything not listed in https://devcenter.heroku.com/articles/ruby-support#ruby-versions is no longer available by default.
For users who are unable to update Ruby to a supported version, our recommendation is to switch your app to use the older
cedar-14 stack instead, which is supported until April 2019. This can be done with the following command:
heroku stack:set cedar-14 -a my-example-app-name