This error occurs when DPD UK cannot create a Synkka return label, usually because the customer's address is in Northern Ireland, which is not supported for DPD returns through this integration.
Error message
"Failed to create Synkka shipping label: Synkka Print Label API Error 422: Unprocessable Entity"
What this means
DPD UK does not support returns from Northern Ireland through the Synkka integration.
How to fix it
Open the RMA.
Check whether the customer's return address is in Northern Ireland.
If it is, change the carrier to Royal Mail.
Regenerate the label.
If the issue continues
Contact support with the RMA ID, customer address, and a screenshot of the error.