The Stage attribute is not available in the new UX in PPM 15.6.1. It is an out of the box attribute and the API ID is grayed out. Does this field reside anywhere else other than on the details portion of the project in the New UX?
Release : 15.6.1
Component : CA PPM SAAS STUDIO
The Stage lookup is not available in the new UX in 15.6.1 because it is a static dependent lookup. This type of lookup is not supported in the new UX on 15.6.1 or any of the previous releases. However, this field is expected to be included in our upcoming 15.7 release (tentatively scheduled to become available some time in September 2019). In 15.7, the Stage attribute will be available to put on a card face as well as a blueprint, but will not be available to add as a column on a board.