While upgrading a client from the Dynamics 365 classic interface to the Unified Interface, I noticed that I could not edit records even though I had System Administrator access. I could edit the same records in the classic interface with the same Security Roles.
The screenshots below show the difference between the two clients.
This is because the Unified Interface renders as “mobile” and the Entity Settings have “Read-only in mobile” ticked.
The simple fix is to untick “Read-only in mobile” and save and publish the entity.