This error happens when your Stamps.com or Endicia account has insufficient funds to generate labels through ShipStation.
Just add funds and you'll be able to retry the label. If you want to learn how to auto-fund your stamps.com account, click here!
If that doesn't solve your issue, you are most likely now receiving a different error message. Feel free to send us the return number and your store URL at email@example.com so we can take a look for you, or take a look at the other articles of our Help Center to troubleshoot yourself!