I just planted two knockout roses and the bottom leaves on one of them are turning yellow. What do I need to do for it?

It might just be transplant shock, so make sure you keep it adequately watered. If you didn't use a water soluble fertilizer to water it in with when you planted it, go ahead and apply a couple of gallons.

It could also be black spot, a fungal disease, remove those yellow leaves, apply a fresh layer of mulch, and keep watching for spots and more yellowing. You'll need to use a fungicide if it gets worse.