Hello Ron Weasley,
Thank you for posting your question in the Microsoft Q&A Community.Based on the error you posted, the actor performing the action doesnt have the permission (Microsoft.ApiManagement/service/users/write) to complete the process. Kindly assign the API Management Service Contributor on the Resource TestResourceSub2 to give the necesarry permission. Alternately, you can create a custom role and add the missing permission (Microsoft.ApiManagement/service/*) or (Microsoft.ApiManagement/service/write)
Let me know if this helps.
Babafemi