Where is your ML skill hosted? Is it in a Cloud AI Center? or in an AI Center installed in our premises? Is consumption supposed to happen from your cloud account? Then you need to add the Document Understanding ApiKey. If consumption if supposed to happen from your on-prem orchestrator, then please let me know what version of Orchestrator you are using.
The skill is hosted on AI Center installed in our premises. We are using Orchestrator version 2020.10.1