Can you put a set monster up for tribute?

Yes. You can Tribute a Set monster. When a card states, "Tribute 1 monster," it doesn't matter if the monster is face-down or face-up. This is also the case for cards like "Monster Reborn" and "Call of the Haunted" that require you to Tribute monster(s). So, you could Tribute a Set monster if you don't have a face-up monster on the field to pay the cost. You can also tribute a face-down monster if you have other face-down monsters on the field.