If you'd like to create a SharePoint group through Teams Manager, the corresponding option has to be enabled first.
Here's how you can do that:
Navigate to Settings > Requests within Teams Manager.
Find the "Enable SharePoint Sites" toggle under the Enable additional content types section and make sure that it's enabled (this is an Enterprise feature, so you might have to upgrade your plan if you don't see it).
Once SharePoint sites are enabled, navigate to Settings > Templates.
Select the newly added SharePoint Site option.
Create a SharePoint Site Template.
Once done, users will be able to create SharePoint Sites based on your Template.
As a user, if you don't see the option for SharePoint Templates, you can contact your organization's Teams Admin for more information.