I have been getting that message a lot over the past year @disturbed1rayray and people post about it frequently on the shipping board. Most of the time I can go to this page and find that the labels were purchased so I use the reprint option.
Reprint is available for 24 hours. When beyond the reprint timeline, you would need to void USPS (or ignore UPS/FedEX) labels which can be done for up to 5 days and purchase a new label.
If the problem is exclusively with printing then the best options are generally:
1. Download the label as a PDF then print
2. Try a different browser
