The most common reason rates won't be returned from DHL Express after you've connected it to SKULabs is due to the DHL information and credentials provided on Easypost.
To resolve this, update DHL information and credentials on Easypost. Make sure the "reseller" box is selected.
Once you have made the update on Easypost, return to SKULabs and try fetching rates to see if that fixes it.
If that doesn't fix it, contact Easypost at [email protected] and inquire as to why you're receiving the error. In your email to Easypost, include the carrier ID and shipment ID provided in the rates error.