Invite admin to SharePoint Content Type Hub site
Global SharePoint Content types can be found via the Microsoft 365 Tenant Admin under content services. For that, we have to assign a user SharePoint Tenant admin. What if we want to grant a user the ability to modify just content types without having to possess SharePoint Admin permissions?
SharePoint Online Admin cannot access the Content Type Hub site
Content Type Hub can also be controlled via a SharePoint site collection. The problem is that you as an admin don't have access to it. You cannot even invite yourself to it from the Tenant Active sites list because it does not exist.
Because the Content Type Hub is special and is not listed within SharePoint Admin Center you can use the following URL to add your SharePoint Administration account as a site collection administrator.
Do not forget to replace TenantName with your tenant name and that should be the job done!
You might get an error when you submit the button, but you are actually invited as an admin. Just open the URL in your browser.
Thanks to Siôn J. Lewis!
Reference: https://www.sjlewis.com/2020/05/12/sharepoint-online-admin-cannot-access-the-content-type-hub/