When you have created a Sharepoint site using new Sharepoint Online admin center, then you can see access deny when you need to upload new content type template for custom documents to the documents library.
It's because scripting on the new, created site is turned off.
You need to:
1.Turn on scripting on the root Sharepoint settings:
2. Execute from the Powershell to turn on scripting for the new site:
Set-SPOsite https://YOURTENANTNAMEHERE.sharepoint.com/sites/YOURSITENAMEHERE -DenyAddAndCustomizePages 0