I was able to find a solution to this problem through Google’s support forum. I am coming back to post it here in case someone has the same problem.

I found out that several hosting companies (including HostGator and Bluehost) install a caching plugin that isn’t shown in your plugins. HostGator’s caching plugin is called “Endurance Cache”. You need to remove this plugin or your AMP content will never be indexed by Google. You can’t disable the plugin natively from within your WordPress settings. Therefore you must remove it from the root. This can be easily done through FTP or file manager.

1) Log into your Cpanel and click on Files.
2) Choose your domain/subdomain and click on the WP-Content.
3) Click on the “mu-plugins”.
4) Rename the File “endurance-page-cache.php” to “endurance-page-cache.php.old”
5) Create a blank file there and name it “endurance-page-cache.php”

Once renamed/removed your AMP pages will work perfect!

Just sent an email with my details. I have all the same settings as you enabled. It isn’t that the AMP URL isn’t working. The problem is that the base (non-amp) URL is throwing up that error. This is preventing indexing.

You can see the error in these screenshots: https://imgur.com/a/d4t3r0P

But you can see it is that same error.

Sorry, I am running version 10.3.6.1 of Newspaper.

That is perfect. I disabled amp/mobile for my homepage but will leave everything else with that optimized version. Thank you.