'User cannot be found' during new app catalog creation

Hello,

I'm trying to create my first app catalog in a Sharepoint online tenant following this kb: Manage apps using the App Catalog - SharePoint - SharePoint in Microsoft 365 | Microsoft Docs

After selected the first option and 'OK', I receive the error "User cannot be found".

I'm logged in as tenant administrator and there are already sites created in it (modern and classic).

What am I wrong?

Thanks