Conditional Access policy blocking GDAP. Customer will need to exclude the partner from the policy, as per document: https://learn.microsoft.com/en-us/partner-center/gdap-faq#what-is-the-recommended-next-step-if-the-conditional-access-policy-set-by-the-customer-blocks-all-external-access-including-csps-access-aobo-to-the-customers-tenant