You don’t have any connections to ground so there is no ground to seed. You need to make a connection to ground in at least one view to get a ground fill (AFAIK, I’m not a ground fill expert!) This should make ground fill work I believe
I’m not sure if you need the trace in pcb to be routed or if the rats nest line will be sufficient, but you need at least the rats nest line to tell Fritizing it should be ground.
Peter