Gift cards starting with INC aren't redeeming on the Spotify.com website.
We are currently investigating this for users. For now please send us an email via the contact form with a picture of the back of the gift card and the receipt. Be sure to include your Spotify username in the email as well.
This is now sorted, please let us know if you still have any trouble redeeming codes. Be sure to try clearing your browser's cache first if you run into any issues. Thanks for your patience while this was fixed.