Have you got a PC or Laptop and tried redeeming the offer by following these instructions linked below ?
I am running out of suggestions to be honest, i guessing your tried this method on your Android Phone ?
Also, perhaps trying going to the Chromecast offers website link below with your Android Phone using the Chrome internet browser to see by pressing the redeem button if the NowTV code shows up on the end of the url address bar where you can manually add the code to your account yourself.
If that fails, you could try nowtv live chat for further assistance from the link page below (click on the drop down arrow below Anything else to reveal the live chat button).
Hopefully somebody from the NowTV Team will help you out.
I just checked my Chromecast and i have a voucher ready to redeem on my account and i tried claiming the offer via the Google Home App on my Android Phone and via my PC and both methods for me appeared to be working.
Don't know why you can't seem to find or redeem the offer, the only thing to check is that the wifi is stable from your Chromecast where you need the white light showing on the Chromecast when attempting to redeem the Now TV offer.
Also assuming your are based in the UK and not the Republic of Ireland or using any VPN on your home network ?
Anyway goodluck and hope you get it sorted soon.
I know Schnapps has mentioned this already but you are trying to redeem this offer on your android phone by using the Chrome browser?
I had a hard time trying to redeem the "2 months cinema for £5" offer, and actually gave up on it, until I used my android tablet with my Chrome browser then the voucher appeared in the URL.
Anyway, good luck.
PS. BTW, my chromecast device, playing Spotify music, and my tablet were both working on my home wi-fi network at the time.