I have tried printing a MyHermes shipping label and am seeing following error returned:

The remote server returned an error: (401) Unauthorized


This error may be returned due to an authorisation issue with MyHermes. In some cases the error can be resolved if you go to > Shipping > Integrations > MyHermes > 
Config and click on reauthorise your account.

In case it doesn't resolve the issue for you, please contact Linnworks support and we will be able to assist you further.