Thanks for providing that x-request-id and insights on the error you are seeing. While we can't provide authenticated support though through these forums at this time, I can pass on some insight based on experience when coming across this type of error. Service is unavailable would generally be an expected error returned by stores on a plan that isn't capable of processing orders, when attempting to create an order via API (ie. a developer or pause and build plan).
My best suggestion would be to first confirm if your client is on a plan that includes order creation as part of the feature set, and if so, have them reach our support service directly for authenticated and more direct assistance - Cheers!