Haven't used my Chromecast in a few years on the NowTV service, however if you can post back here what the error message says then perhaps somebody can give you some further assistance.
Also, another quick question are you based in the UK or the Republic of Ireland ?
Sorry to hear about the issues you are having when trying to use Chromecast.
Are you able to let us know if you are trying to connect to a Chromecast device or do you have Chromecast built into your TV?
If it is built into your TV, can you please let us know the make and model of your TV?