I can’t create OSSIAs in Elements either?
From the manual:
In Write mode, select the range above/below which you want to add an ossia staff.
If you want to add a two-staff ossia, you must select both instrumental staves.
Add an ossia staff in any of the following ways:
Choose Edit > Staff > Create Ossia Above. …(NOPE)
Choose Edit > Staff > Create Ossia Below. …(NOPE)
Sorry you ran into this - there are different manuals for Pro, Elements, and SE, but if you’re coming from a google search it’s quite possible you’ll land in the Pro manual (especially if you searched for something like adding ossia staves, which doesn’t appear in the Elements manual).
You can find the Dorico 3 Elements manuals here (webhelp and PDF versions, the available languages, and other useful documents).