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)