NeophyteWould suggest you get postman up and running and vet out all the calls you are trying to make before you try to code them in. That will eliminate the question of whether it's something about the call or Posh. Just a suggestion.
That said, when i did your call, the $($DS_CPU.id) variable is the id (9512), not the dataSourceId (24131557):
And my $($inst_CPU.id) was the id (146237749), not any other id. That one stumped me for a while until i learned that lesson.
So, my final call was {{url}}/device/devices/9/devicedatasources/9512/instances/146237749/data
Where my $($device.id) is 9, $$DS_CPU.id) is 9512 (id) and my $($inst_CPU.id) was 146237749.