It will not allow me to kill the bottom coffin. What does this do? Please help.

If you mean in the lower tombs of the graveyard during the hunt for Bodhi, you need to find the third vampire guard. He is downstairs, but will disappear upstairs when confronted and reappear in the round spikey room. Kill him,(watch for traps) and then stake the third coffin. Then prepare for Bodhi...