I have a URL for my Heroku app here: https://damp-sands-09329.herokuapp.com/.
When I go to https://github.com/new/import, I am told to enter \"Your old repository’s clon
Github is correct, what you provided is not a valid Git repository URL.
The Heroku site or web URL (of the form https://xxxxxx.herokuapp.com/) is where your website is deployed, but that's not where the source codes are stored.
What you want is the Heroku Git URL. You can get this in 2 ways:
From the Heroku dashboard
Login into your Heroku dashboard (https://dashboard.heroku.com), go to your app, then go to Settings, then scroll to the Info section. It's the one specified as the Heroku Git URL and is of the form https://git.heroku.com/xxxxxxx.git.
From the Heroku CLI
If you have Heroku CLI installed, you can use the apps:info command to get the Git URL.
$ heroku apps:info app-name
=== app-name
...
Git URL: https://git.heroku.com/app-name.git
...
Web URL: https://app-name.herokuapp.com/
Notice that it shows the Git URL and the Web URL. Github wants the Git URL.