I assume you ask "why Death allowed Knight to delay his death if he'd told him:
there's no delaying this"
To which my answer is: That's not what Death said.
First, Knight knew that Death had been following him.
Death: I have been for a long time at your side.
Block: This I know.
Block: Wait a moment.
Death: You all say that. But I give no respite.
Death doesn't give respite. Now for the magic of the movie: Time is non-existent in the movie. Look at how it begins:
there was silence in heaven about the space of half an hour
And just before Death appears, there is silence. Pay attention to how long it lasted (hint: it's about background sounds). Was it for 30 minutes?
Third, Death is seen "personally" being responsible for the death of a person, so we know it didn't have to wait for a person to "die of something".
And fourth: Knight was dead from the beginning. The whole movie is a Dance Macabre, which we are only able to see through the eyes of Jof at the end.