A cabin crew seat. If it was not in the MEL and there was not a LAME rated on type available then yes it would be normal to have to fly one in. Even though the seat may not have been fixed it may also not be so unusual to require some maintenance activity prior to release. It all depends on how the manuals are written.
The rated on type LAME is the key, - without one of them some things just cannot happen.
I would have thought it may have been possible to operate the flight on a reduced pax/reduced crew scenario but again some things are not always as they appear. You would have to imagine that there was more to it all.