Are you typing your ID in the web version? I saw that some players are having problems with the version of the game where you need to download it.
Try entering your ID where the code is placed and not putting your roblox profile (if you are doing that)
I had no problems doing it on mobile. Hope you get it now. You can use the "Skip" button to play the game faster if you've already done it and couldn't redeem your code.