Error message appears when adding all used dependencies of one report to create update package in Strategy Object Manager 9.x as shown below:

Object with ID 'xxx' and type 56 (Drill map) is not found in metadata. It may have been deleted.
This is caused by an invalid reference in a template object. The template still depends on one drill map which has been removed.
In order to track down the cause of the behavior, customers encountering this issue should make note of application-related changes, Object Migrations, or error messages prior to experiencing the problem. Change Journal, if it has been enabled, can help in preserving this information.
Contact Strategy Technical Support on how to remove such inconsistency