Manage detailed site and subsite creation settings in the classic SharePoint admin center
In the left pane of the new SharePoint admin center, select Settings. At the bottom of the page, select classic settings page.
Under Site Creation, select to show or hide the Create site command.
If you select Show the Create site command, specify the type of site that users can create.
A new team site or communication site: Select to create the group-connected team sites under (/sites or /teams) and whether a secondary contact is required. To let users create sites from a custom form you’ve created, enter its URL in the Use the form at this URL box. When users select which type of site they want to create, they’ll be able to access the form by clicking “See other options.”
A classic team subsite: Use this option to let users create only default classic sites or sites from your custom form. Specify where sites are created, and whether a site classification or secondary contact is required.
Under Subsite creation, on the Site contents page, to create a new subsite, specify whether users can select New > Subsite.