Steam says “Invalid Product Code”.
Check every character: a 0 looks like an O, a 1 like an I, an 8 like a B. Type the code in capitals, with its dashes. The field above removes stray spaces for you.
Steam says the code has already been activated.
Each code can be activated only once. If NeoPiano is already in your library, you’re all set. Otherwise, keep your card and proof of purchase and contact the store you bought it from.
Steam says I made too many attempts.
After several rejected codes in a row, Steam temporarily blocks activation on your account. Wait about an hour, then try again, checking the code carefully.
Do I need one card for Windows and another for Mac?
No. The license belongs to your Steam account: you can install NeoPiano on Windows and macOS with the same account.
I don’t have a Steam account yet.
Create one for free on store.steampowered.com, install the Steam client, then activate your code. It takes a few minutes.