You can return to Google for a refund. After that don't call Google because they won't help. I have heard people getting help from Spotify, but that is just a rumor, my experience was repeated requests for help before they ended up saying the offer was expired after over 10 correspondences.
I cannot believe how badly Spotify have stuffed up this promotion. I upgraded my subscription in good faith and folloed their instructions and have had my order cancelled by google. I have transcripts of text conversations with spotify promising the issue will be sorted. I was told... "Don't worry! You've already secured a device and you're 101% eligible for it."
I've since contacted them again to sort it out but nothing yet. Quite frankly I'm beginning to feel that I've been conned.