No spaces on the end.. just double checked.
If I change the netmask it's allowing me to add the rule. :s
[appadmin@cppm]# network ip add mgmt -d 10.0.0.0/8 -g 172.20.50.1
ERROR - Failed add route for destination=10.0.0.0/8 via=172.20.50.1
[appadmin@cppm]# network ip add mgmt -d 10.0.0.0/9 -g 172.20.50.1
INFO - Added route for destination=10.0.0.0/9 via=172.20.50.1
INFO - New ip rule created with the id = 12002