Hi P, Pioline (Cognizant) Thanks for reaching out. The expression should be Claim.diagnosis.diagnosisCodeableConcept.coding.code, not Claim.diagnosisCodeableConcept.code.
The 424 Failed dependency error usually indicates that the server was unable to complete the request because it depends on another action that has failed. In this case, it could be due to the incorrect expression value. Please try with the above given expression value and see if it resolves your issue.
Let me know incase of further queries, I would be happy to assist you.