I'm getting the exact same error as described here: Can't create job on GCP Cloud Scheduler
and I have been using a service user that I myself created and gave the appropriate permissions. and fully stripping and restoring those permissions yielded no results either.
Cloud scheduler itself must have a service account [1] which is Cloud scheduler Service Agent role [2], we need to check that this particular role has for the Cloud scheduler service account. Also the document [1] could be helpful for better understanding.
Also, please check the documentation [3] for supported regions and If the project does not have an App Engine app, we need to create one.
[1] https://cloud.google.com/scheduler/docs/http-target-auth#setting_up_the_service_account
[2] https://cloud.google.com/iam/docs/understanding-roles
[3] https://cloud.google.com/scheduler/docs/#supported_regions
