It wants your bank details so that it can take the charge for a second month if you don’t cancel out of the voucher you have before the end of the month it covers. And likewise for subsequent months.
Why it declines you because you had (have?) a previous account with the same bank details I can’t tell you, but there are several possible reasons.
Go into Help, more or less at random will do (though starting by re-expressing your current query may be helpful), and there will be a link to the Chat service.
Assuming you get answered on it, and don’t get suddenly cut off while carefully phrasing a long explanation of the issue, as I did twice, you may be able to get a resolution, like maybe getting the other account deleted, or some such.
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.
