Be more specific what exaclty is not working what errors you get and so on.
Here is an example of traffic monitor, it works as expected:
$ curl -k -u admin: -X POST 'https://10.155.101.214/rest/interface/monitor-traffic' --data '{"interface":"sfp12","duration":"2s"}' -H "content-type: application/json"
[{".section":"0","fp-rx-bits-per-second":"45176","fp-rx-packets-per-second":"49","fp-tx-bits-per-second":"222160","fp-tx-packets-per-second":"30","name":"sfp12","rx-bits-per-second":"45176","rx-drops-per-second":"0","rx-errors-per-second":"0","rx-packets-per-second":"49","tx-bits-per-second":"222160","tx-drops-per-second":"0","tx-errors-per-second":"0","tx-packets-per-second":"30","tx-queue-drops-per-second":"0"},{".section":"1","fp-rx-bits-per-second":"24184","fp-rx-packets-per-second":"27","fp-tx-bits-per-second":"214616","fp-tx-packets-per-second":"31","name":"sfp12","rx-bits-per-second":"24184","rx-drops-per-second":"0","rx-errors-per-second":"0","rx-packets-per-second":"27","tx-bits-per-second":"214616","tx-drops-per-second":"0","tx-errors-per-second":"0","tx-packets-per-second":"31","tx-queue-drops-per-second":"0"}]