When changing the interface of Dynamics 365 to Unified Interface, if by any chance you don’t see the New Button to create entities, or even if the entities are read-only even being System Administrator, only thing you need to do is:
– Go to Advanced Settings like in the image below:
– Then, go entity by entity and uncheck the following setting.
The Unified Interface act as Mobile version apparently, even accessing from the browser in your desktop or laptop. Publish settings and should work.
Hope it helps.