It also lists ONTAP versions, which 8.3.1 is listed as supported...ONTAPI 1.30 was released with 8.3.0 and 1.31 was released with 8.3.1, so it's not 100% clear which is accurate.
That being said, NMSDK is primarily an enabler for accessing ONTAPI/ZAPI, so it *should* still work. For the API you're specifying, are you executing against the Vserver/SVM, or against the cluster? "net-routes-create" should be directed at the SVM.
Hope that helps.
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