Hi kristasoft,
Can you make sure the service account that you are using has the right permissions and also you are downloading and using the right JSON key from your google play store console.
If you are not sure, I would encourage you to go ahead and assign admin-level privileges to the service account. Then download a fresh copy of the JSON key and then try creating a new destination for the user.
If you are facing any further issues, We would need to take a closer look at your account and it would be helpful if you can raise a ticket with your appflow app ID here Appflow support for further assistance.
Thanks,