Full error:
"The Marketplace ID being returned is an International Amazon selling platform ID. You must sign-in to Amazon using a US Marketplace ID to connect that account to InventoryLab. Currently InventoryLab only supports the Amazon US selling platform but expansion to International platforms is in development."
The issue appears to be that your primary selling account is tied to an international marketplace, so when you set up your first account it was tied to something other than the .com but the .com marketplace was added later.
You can try to change this within Seller Central before you try to bind the InventoryLab account to it. Log in to Seller Central and at the top of the page you will see your marketplace displayed. Click on this and in the drop down change that to the US marketplace:
Then come back to InventoryLab, log out, log in, and follow the bind steps to see if that resolves this issue.
If it does not, then contact Seller Support and see if they can change your primary marketplace to the .com from whatever marketplace it is set up with as primary. You may have to contact the MWS Team for assistance with this issue.
If they cannot do that, you would have to create a new seller account with .com as the primary selling marketplace before being able to use our application.