Hi team,
I received some cases related same issue:
I cannot add my custom entity to Data export service.
Cause:
change track is disable to custom entity.
Solution:
1)Go to Customizations > Customize the System.
2)Select an entity, and under Data Services, select the Change Tracking check box.
more details Dynamics Data Export Service, Aka DES , cannot enable custom entity, gray #drops - Microsoft Dynamics CRM Community