Welcome to the forum.
The Chromecast will take up one of your device slots, which if you've already used up your allowance is causing the issue. If you have four devices already registered you will need to go into your My Account - Manage Devices page
and delete one of the devices you want the Chromecast to replace (if you only intend to use your phone as a casting device and not for watching Now TV on separately you can remove that if it's listed). Then try again.
Otherwise if you still get stuck, or you have less than four devices already then you will probably need a device reset.
In this case you could have a word with Live Chat, explain what's happened and see if they can reset your devices for you. To get there go to the following help page
and click the green Contact Us button near the bottom left of it (not the How to Contact Us link right at the bottom as that takes you somewhere else) then choose the Live Chat option on the following page.
The android phone is the remote controll of the chromecast. the chromecast is the playing device as soon as the remote selects content to play on the chromecast, the chromecast registers on the NowTV servers...
You need to Enable to controll the chomecast every time you launch the app... thats the bug bear of chromecast... you have to continuely select to connect to chromecast to controll it when viewing via chromecast...
Every time you launch the app...
Thats only if the remote (Aka Phone/tablet so on is not allready got flat battery...)