Did you taste the beer before adding coffee? If so, did it taste good? That would determine whether the problem is just the coffee addition, the beer, or both.
I have no idea what that kit should taste like but I suspect the problem lies in the way they suggest adding the coffee. That adds a lot of extra water, which will thin out the beer, and drip brewed coffee generally produces the most acidic coffee. (Better options include steeping the beans directly or making a concentrated cold brew coffee. Avoids both of these problems in the future but doesn't help with your current beer.)
For the current beer, options are somewhat limited. If it's kegged you could lower the pressure for more of a cask-like pour which will mask some of the thinness and acidity. If you haven't packaged it then you could target lower carbonation for the same reason. Adding lactose to improve the body may help. If bottled, there's not too much you can do. Maybe a little chocolate syrup in the glass.