Have you verified that there is a client called api.client in your SAS Viya environment? Also, is the URL you posted below the call you sent or did you make it generic for posting. sasserver.demo.sas.com is a generic URL I made up for documentation purposes. You would need to update the URL and client id with something specific to your implementation.
I made it generic for posting. I don't think the api client and api secret were set up which might explain why it isn't working as intended.
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