If you installed the 8.51 webservices_rest package, then I would expect that you would run into problems. The following Note can be found on the RESTful Web Services Wiki page found at the following link:
https://docops.ca.com/ca-unified-infrastructure-management-probes/ga/en/probe-development-tools/restful-web-services
Note: The webservices_REST probe is updated to run with the latest CA UIM database schema. When updating UIM, be sure to download the webservices_REST probe with the corresponding version number. Do not download webservices_REST with a later version than your CA UIM: doing so may break endpoints.
Make sure that you have installed the 8.47 webservices_rest package. If you have and you still cannot get your alarm REST calls to work, I would recommend opening a case to look into this further.
I hope this helps.