This is quite common, you have a cPanel (or others) vps and disk usage on accounts shows "Unlimited". You try with "Initial quota setup" or /script/fixquota and get an error like this:
No filesystems with quota detected.
Solution is to enable second level quota on the node. To do it execute these commands on the node:
vzctl stop CTID
vzctl set CTID --quotaugidlimit 100 --save
vzctl start CTID
Then, if you're under cPanel, do the "Initial quota setup" thing or run /script/fixquota
CTID is the container/vps id number.
On the second command, --quotaugidlimit 100 <- this is the number that defines the max file owners. Example: you have 100 users into the vps, set it to 120. You have 300 users into the vps, set it to something more than 300. I've used 100 on this example because it's a common average value. Keep in mind that the more you set, the more kernel memory overhead will be created.