There are a few possible causes for this error.
1. The item you are trying to list is not in the Amazon catalog.
Visit Seller Central and try to Add the Product. If you are unable to find the ASIN/UPC, it is not yet in their catalog or has been changed. You can add the item to the catalog directly through Seller Central or your can contact Seller Support for additional information.
2. Your session has timed out
InventoryLab will log you out if you are idle for a period of time. This typically will occur after 2 hours, however, this action is not always apparent. Click on your name in the upper right hand corner and Logout of InventoryLab and log back in.
3. You are no longer a Pro Account Seller.
A minimum requirement of InventoryLab is that you must have a Pro Seller Account with Amazon. Check your Amazon Seller Account to make sure your account level is sufficient.