Try cleaning the disk, first with a can of air, then if it still doesn't work try a fresh microfiber cloth and azeotropic alcohol (aka Everclear, 190 proof/95% ABV). If that still doesn't work, you can always try to use a disk resurfacer. Warning, disk resurfacers can make the problem even worse if the equipment malfunctions (which some are prone to do after a distressingly small number of uses), if you don't know what you're doing, or you use a poor buffer pad. Note that disc resurfacers will not help if the problem is label-side damage or if you have a dirty lens.
While we're on the subject, make sure your lens is clean. I'm not sure how you'd do that, but I'd advise you don't take apart your Wii U to find out. I recently had to replace my PS1's spindle hub and it was a complete nightmare to reseat it properly to minimize load times and read errors. Keep in mind that CDs have a much, much lower tolerance than the proprietary format the Wii U uses and my repaired PS1 still won't play FMVs without a LOT of stuttering.
Don't buy digital. When the servers get shut down, and they will eventually, you can only hope your Wii U's SSD never goes bad or you'll lose all of your digital games.
