Warning: This guide intended for Rails 6.1. (It may not be complete)
This is direction for Migration from c9.io to AWS Cloud9 or to simply setup AWS Cloud9 for Rails Development, after you follow the cloud9 migration directions and created a workspace, keeping in mind the Workspace settings Section below.
Be sure you are using the right version of rails. You may need to remove rails and rail-ties and install rails 6.1. Check online for how (search how to reinstall rails with a specific version)