You can... stop the array, click the Add Pool button, set it with 1 slot, assign the SSD to that slot. Then do the format option that will appear in the Array Operations section after you next start the array. And FYI, unRAID ships with a base section called Unassigned Devices, for any devices that haven't been assigned to the array or a pool. To make the Unassigned Devices (UD) section more useable, you need to install the UD, UD+ and UD Preclear plugins from the Apps. tab. Then enable `destructive mode` in Settings --> User Utilities --> Unassigned Devices.
Progress made: I don't know if it was just coincidence, but the SSD I have installed was not formatted. After formatting, share creation worked. Now my other problem is why I can't use my SSD as a cache drive! Forum post about that here:
Ruckus42's post in NVMe M.2 Drive shows as unassigned - need as cache drive was marked as the answer