A subtenant users needs to be able to see an existing asset.
Solution
A subtenant user can only access assets that are explicitly assigned to their subtenant. In order to make an existing asset visible within a subtenant, the tenant admin needs to assign the asset to the subtenant with following steps:
Notes
Additional information about Subtenant management can be found in following documentation page:
https://developer.mindsphere.io/howto/howto-subtenant-management.html