Camelot (Pioneer54 vs. skinny18)

This game is ongoing, but well beyond the position linked here. I tried to move 9.Ni7-i9, but the system would not allow me to stop and submit the move from that point. Why? I had no jump, so any piece could have been moved, and cantering is supposedly optional.

Is this a site bug, rule ambiguity, or something else I missed?