Can you tile over wallpaper

If you are thinking of Ceramic or porcelain tile the quick answer is NO!!

The moisture in the adhesive/thin set will likely make the wallpaper release at the worst possible time. It needs to come off & the substrate needs roughed up a bit to give the thin set something to mechanically bond to.

If this is a wet area like a shower or tub surround DRYWALL is a NO NO too, should be green board.