ISV (Independent Software Vendor): A Microsoft program that helps customers adopt Microsoft Cloud solutions and drive user adoption.
I have the same question!
The authorization token included in the request is valid, however it does not include the correct audience or the entra app id.
https://learn.microsoft.com/en-us/partner-center/marketplace-offers/pc-saas-registration
Target resource for which the token is requested with a default scope. Use 20e940b3-4c77-4b0b-9a53-9e16a1b010a7/.default because Marketplace SaaS API is always the target resource in this case.