I have to admit, that this is not a scenario I have tested or verified. But my best guess would be that you are not allowed to create a subscription as a guest account.
It's not clear by your description if you created a new local user in the new tenant or if you are using your own user from tenant 1 for this procedure, but I assume that you are using your own account from tenant1. I can see two ways to workaround this - and the best way basically depends on your payment method.
- Create a local user in the new tenant, provide the user with the correct permissions and see if this user can create the subscription (which is should be able to provided it have the correct permissions)
- If you are using your guest account from tenant1: Go back to tenant1, create the subscription there , then open the new subscription and press "change directory" and point to the new tenant, and after that the subscription is living in the correct tenant.
Solution 2 requires the user to have permission in both tenants.