What is the model number of the Samsung TV?
Basically, you need to go into the Settings, probably System, and look for the option to update the TV’s operating software, but how exactly you reach this depends on the model year of the TV, as included in the model number.
But have a poke about in there, see if you can find it, and if there is a system update, apply it, and then try signing on to Now again.
If there is no update available, then possibly the TV software has gone a bit sideways; unplug the TV from the mains, hold down the physical on/off button on the TV for 30 seconds, wait a further two minutes, and then plug it back in.
No guarantees, but these are the first things to try.
Set a Payment PIN on your account so that no-one but you can buy memberships on it. Check your bank accounts monthly for any other unexpected payments to Now. That way you can at least nip them in the bud, while you and Now figure out whose fault they are.
If you take an offer that comes with a ‘free trial’ of Boost or another Membership, Cancel them straight away. You will still get the full duration of the trial, and if you decide you do want to continue with one or another of the trial products, just revisit the Cancel, and UnCancel it.