Why does check engine light stay on after replacing the egr valve on a 2001 buick lesabre?

Because you did not have a faulty EGR valve. You misdiagnosed the problem. Start over and remember the code only tells you what part is being effected not necessarily the defective part. This is assuming you reset the light or drove the car enough cycles for the light to reset. If not then reset the light and see if it comes back on.