The error (DHL Validation Failed: Error code: 420506 Data: Postcode not found. Expected formats:) occurs when the postcode or country information is incorrect.
To resolve this issue, follow these steps:
Click Orders, then Overview.
Find the order.
Click Actions, then Pick Order.
Click Edit Address and verify that the postcode and country are correct.
Once the above steps are completed, click Create Shipment, and the error message should no longer appear.