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.
Release : All Supported
Component : CA PPM SAAS STUDIO
This is working as designed. Sub objects of cost plans are currently not available for API Enablement.
The following subobject types are available for API Enabling as referenced at: API Enable Custom Objects and Custom Investments