Question

But, in a larger sense, we can not dedicate -- we can not consecrate -- we can not hallow this ground. The brave men, living and dead, who struggled here have consecrated it, far above our poor power to add or detract. Means:

Answer 1

But in a bigger picture we can not make this place holy because the men died here already did

Answer 2

Lincoln is saying that the actions of the living will never be able to equal the sacrifice of the dead.