Custom master pages are currently not supported for "modern" pages.
You can find supported/unsupported customizations in below documentations:
Customizing "modern" site pages
Customizing "modern" team sites
Branding your SharePoint site
There is no SharePoint out of the box option to hide office 365 ribbon at the top.
You are missing PermissionsString in your code.
As per the reference you provided, you can add any permission type from the below list based on your requirement:
Read more about SharePoint Base Permissions at SPBasePermissions ...