There are two reasons why you see a 403 error:

1. You used Server container GTM ID when setting up a custom loader. 
2. You did not publish the init version of the web container.

SOLUTION:

1. If you use a custom loader and see a 403 error when opening the sGTM container URL, it might be because you've used the server Google Tag Manager ID when setting up a custom loader. You must add your WEB Google Tag Manager ID.

To check the custom loader setting log in to stape.io account -> open sGTM container -> click Power Ups -> open custom loader and check if you added WEB GTM ID. 

Change the container ID to the WEB container ID, click save, and update the GTM script on your site.

To find your Web container id, you need to open Google Tag Manager and select your web container. There you will be able to see the web container id.

2. You did not publish the init version of the web container.

Open your web Google Tag Manager container and ensure that it's published and running.