/admin/custompage (and /admin/customemail for email templates) is a source-code editor reserved for Sprwt super-admins. Sprwt staff use it to write client-specific templates, modules and styles for your site, with a draft/Publish workflow (Remove draft discards unpublished changes), a way to Clone an existing module into a custom one, and a Documentation panel of placeholders. Business admins cannot open it — if you need a bespoke layout or module that the standard modules can't achieve, request it from Sprwt support and it will appear in your page's template picker as a (Client Custom) template, or as a normal module in your module library.