The locked status error message generally is the result of one of two things:
you processed the ShipmentID referenced in the error message in Seller Central before clicking FINISH SHIPMENT in ScanPower
you deleted or archived the ShipmentID referenced in the error message in Seller Central without also archiving the shipment in ScanPower.
or an issue on the Amazon side which moves the shipment out of WORKING status without you triggering it. If you feel like this is the issue then you need to open a support ticket with Seller Central, report the issue, and ask them to toggle the shipment back to WORKING status.
To find out which is the reason, please log into Seller Central, access your Shipping Queue and locate the ShipmentID referenced in the error message. What is the status of that shipment?
If the shipment has already been processed for shipping in Seller Central then please go back into ScanPower, click LIST, locate the shipment icon for the ShipmentID referenced in the error message, open it and click FINISH SHIPMENT.
If the shipment was archived or deleted in Seller Central then please go back into ScanPower, click LIST, locate the shipment icon for the ShipmentID referenced in the error message, right-click the icon and select ARCHIVE.
This should resolve the issue and permit you to continue listing.