Hi Susan! Its certainly a strange issue. Booting to recovery and using the firmware disk utility *should* allow you to erase the startup volume and reinstall MacOS as in recovery the internal SSD isn't being used.

There are some things you can try though. Are you comfortable using the Terminal? Accessible from the recovery menu bar, you can use the command line to force unmount a disk. If you type in

diskutil list

it will bring up a list of available drives. Make a note of the identifier of your internal drive. Then type

diskutil unmountDisk force /dev/xxxxx

where xxxxx is your disk identifier (disk0s1 or similar)

Try erasing from disk utility after this. If you have another Mac knocking around you can use target disk mode. This enabled one Mac to access the drive of another. You can find a guide specific to your Mac pretty easily with Google. If you have a spare external drive you can also install MacOS on that and use that to access the erroneous internal one.

And the last resort? If your Mac is portable enough see if the Genius Bar at your local Apple Store can offer any assistance.

Let me know how you get on :)