Royal Mail loads a limited number of barcodes to every service on your Royal Mail account. As a result, you may receive "Shipment numbers are exhausted, so new shipments cannot be created. Royal Mail is aware of this problem and will be uploading new barcode prefixes shortly. Error code: E1149" when you have used all of the barcodes allocated to any specific service.
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 add more bar codes to your account.
If in doubt, please get in touch with Shiptheory support and we will be happy to help where we can.