Symptom
"External ID" field is not available in Workflow Rules of type "Field Update" for the Account Object.
Environment
SAP Cloud for Customer
Cause
Account's External ID can't be updated via Workflow Rule, the External ID is stored in the BO ObjectIdentifierMapping.
Resolution
This is the expected system behavior.
If you need a specific custom logic to fill the External ID we would recommend the PDI Community (https://community.sap.com/topics/cloud-applications-studio) where you can meet many ABSL Developers from both SAP and Partner Environments who are willing to help in such queries.
An alternative option may be to check the SAP Cloud for Customer Influence Site to submit an idea.
Keywords
External ID ; Account ; Customer ; Field Update , KBA , LOD-CRM-ACC , Account , How To