I was having trouble with my Paypal Here codes as I was scratching them off too hard and thus smearing the residue which was on top of the scratch off code. It seemed like I was unable to redeem it since I scratched it off too hard. But, I was able to remove the residue and finally reveal the hidden code and redeem the product.
Here is what you have to do if your code is unreadable because you scratched too hard or have a hard time reading the redemption code since it is a gray color.
Take some hand sanitizer and smear it on top of the scratch off section, and a reaction should take place which will remove all the film on top of the code and should make the area clear and the code easily readable.