Yes, this is a temporary glitch... try signing out and back in to My Disney Experience, as that seems to reload everything.
I ran into this today while trying to actually cancel a reservation (I booked an insurance reservation for Jiko at the 6am opening, then scored a Wanyama Safari reservation when that opened up at 9am... so I needed to drop the dinner-only ADR).
When I clicked cancel on my reservation, it gave me this "cancellation unavailable" message... which stuck around until I logged out and back in. Then a totally different set of reservations said 'cancellation unavailable', but at that point I was able to easily cancel.