Halo 4 Final Mission Bug

There seems to be a bug at the very end of Halo 4. When facing the Didact, when you climb back up onto the light bridge and run at him your suposed to plant a grenade on him, but every time i do it says “RT To Fire Machine Gun” and wont let me plant a grenade. Im caught in an perpetual loop. It is very frustrating. I have tried starting the level over again, the WHOLE level, at least a dozen times.

This needs to be looked into please.

Thanks,

Erin