Why does Build Disk fail with an error about not enough space/extents when the UI reports there is plenty of space for my disk?
Check the Alerts page. There may be Zombie disks there, which need to be removed.
Also, the value set within the OnApp UI for the datastore may have been set incorrectly. To check that, log into a Hypervisor and run 'pvscan' that will show you the real size of the data store. You can edit the Data Store via the OnApp UI, if the current value is incorrect.