Royal Mail may return the message "serviceOffering (also known as Service) is not enabled for this account. Error code: E1090.". if you are attempting to send parcels on a service code that you do not have enabled to on your Royal Mail account.
For example, if you tried to ship on the ROYAL MAIL 24 (P) FLAT RATE service and saw the above message, this means that your Royal Mail account is not setup to use ROYAL MAIL 24 (P) FLAT RATE.
The error message is coming back directly from Royal Mail, Shiptheory is unable to dispatch orders on your account for that specific service code you have attempted to use.
Please contact the Royal Mail help desk on 08456 047267 and provide them the message exactly as above, along with the service code you have attempted to use and they should be able to advise the best course of action.
If in doubt, please get in touch with Shiptheory support and we will be happy to help where we can.