Don't know about the REST API (syntax error) thing, but I am just wondering is it b'cos of the fact that 'aggr' is not assigned to SVM as you guessed it ?
Is that aggregate assigned to the SVM ?
::> vserver show -fields aggr-list
Try to add that aggr to the SVM and try the command again?
::> vserver add-aggregates -vserver svm1 -aggregates ONTAP912clst_VM_DISK_1