In case this helps anyone, I’ve had issues setting up the rails source code. Here were the solutions:

In Gemfile, change

gem 'rails'

to

gem 'rails', "~> 6.1.3.1"

Then run

$ bundle update rails

Then change

gem "activerecord-postgres_enum"

to

gem "activerecord-pg_enum"

and run

$ bin/setup

Then, I needed to jump into the three view files: box.html.erb, intro.html.erb and page.html.erb and remove a LOT of these. 38 in total:

<%

which seem to appear before and after most elements.

Finally, I needed to delete the .browserslistrc file as it was conflicting.

THEN, I was able to run

rails server

and see the app.

I hope this helps someone and fingers crossed these changes can be made to the files to make it a bit easier for the next person.