I have tried printing a Spring Omni label and am seeing following error returned:
Invalid input : Spring Clear setup incomplete
This error usually is returned for IOSS orders when 'Customs Value Includes Tax' service option is enabled, meaning that the order total submitted to the courier (including tax) may exceed the 150€ threshold and become incompatible with IOSS.
Please go to Linnworks.net > Shipping > Integrations > Spring Omni > Services > Edit screen and disable that setting to ensure that the customs value submitted to Spring would not include the tax in it. This will ensure that the customs value will not exceed the above threshold.
If the issue persists, please contact Linnworks Technical Support team for further assistance.