There are no options for the Disk uuid box in the preform benchmarks page.
OnApp 5.x / 6.x
Something that is implied but not mentioned in the documentation here https://docs.onapp.com/display/IS/Create+Performance+Benchmark is that the benchmark test needs to be performed on a zombie disk, or a disk that is not attached to any VM. If there are no disk options you should create a disk in that datastore via the datastores page. Then you will have an option for disks in the benchmark page.