404 error for Google Tag Manager

My issue was that had AdBlock enabled.

I had the same issue even after publishing and got it fixed by simply deleting the subdomain "www" from the tag manager domain "https://googletagmanager.com" instead of "https://www.googletagmanager.com"

Seems the problem is that the DNS is blocking www. or the subdomain doesn't reroute properly to our region!

You need to publish a version of your container. If it is not published, the request will return a 404 error.

To publish your current workspace:

1. Click Submit at the top right hand side of the screen. The Submit Changes screen will appear, with options to publish the container and save a version of your container.
2. Select Publish and Create Version if it is not already selected.
3. Review the Workspace Changes section to see if your configuration appears as you expect.
4. Enter a Version Name and Version Description.
5. If you have Tag Manager configured to use multiple environments, use the Publish to Environment section to select which environment you'd like to publish to.
6. Click Publish.