Share point - create a new App / App Credentials

Hi all, 

I must create a new app in my share point.

I go on page https://Dominio.sharepoint.com/_layouts/15/appregnew.aspx

But when I compile all form data, it returns a generic error.

I'm using standard Sandbox 365.

I enabled SiteOwnerManageLegacyServicePrincipalEnabled parameter.

Can you suggest me how to inspect the error received?

Thanks at all