It's important you register with the correct workflow server - likely prod-workflow.reltio.com, and you also need to make sure you include the correct EnvironmentURL in the header. I am unsure what permissions you need to perform this registration. I did it a few years back, but most tenants come registered now. If the below script does not provide your desired results, you will just need to submit a ticket to support and request that Reltio DevOps register then tenant for you.
You also need to make sure that your user credentials are explicitly granted ROLE_WORKFLOW. Even if you are have ROLE_ADMIN_TENANT you need to explicitly grant yourself ROLE_WORKFLOW to perform many workflow related tasks. Also note that in the past the Workflow servers cached user's role assignments, so adjusting your permissions could take as much as an hour to get refreshed from the tenant privileges.
curl --location --request POST 'https://prod-workflow.reltio.com/workflow-adapter/workflow/registrations' \
--header 'Authorization: Bearer yourAccessToken' \
--header 'Content-Type: application/json' \