About the required property items on Modern UI Document Library
I hope that the files which uploaded to document library can only be uploaded after entering the necessary items.
On Classic UI, by enabling the “content type” and turning on the check-out function, the above operations can be achieved.
On Modern UI, even you make the same settings, the uploaded document is uploaded as it is, and it is necessary to submit the required items later by manual.
I hope that also on Modern UI, when the same settings has been made, the pop-up page with the required property items can appeared and be written.