I am trying to API enable a sub-object of the Cost Plan object so that I can use it in the Modern UI / New User Experience and get error:
ODF-0248: Cannot API enable custom sub-object.
This is working as designed. Sub objects of cost plans are currently not available for API Enablement.
Release : All Supported
Component : CA PPM SAAS STUDIO
The following subobject types are available for API Enabling as referenced at: API Enable Custom Objects and Custom Investments