Unable to add custom template to a custom site content type in Group team site ...

Hello. When I try to 'Upload a new document template' (word document), to a custom content type on my Group team site, I get this error:

"Sorry, you don't have access to this page"

Am I doing something wrong?

In a normal Team Site, I can upload a document template to a custom content type. Therefore, I suspect that it is something peculiar to Group team sites. Odd because I created the Group and am the Site Owner.

Thanks to the following posts / articles, I am now investigating the 'Content Type Hub':

https://techcommunity.microsoft.com/t5/Office-365/Using-custom-site-content-types-and-columns-in-Group-teams-sites/m-p/27417#M1071

http://sympmarc.com/2014/11/25/hidden-content-type-hub-on-office365-tenants/

https://office365.uservoice.com/forums/286611-office-365-groups/suggestions/17118586-office-365-groups-content-type-hub-connection

https://techcommunity.microsoft.com/t5/forums/v3_1/forumtopicpage/board-id/SharePoint_General/message-id/4601/thread-id/4601

In the hub, I have been able to create a custom content type and upload a custom template. However, it is not available in my Group team site yet - now 30+ minutes ago. And I have read recent comments indicating that this can take hours! In addition, my custom content types are specific to this Group and do not need to be available across all Group team sites - they don't really need to be in the hub.

Any ideas how I can upload my custom template directly into the Site Content Types of my Group team site?

Thanks.