Nobody answered yet? Tut.
Do you have a house name rather than a number, or a low numbered house?
Try adding a number to the name, or some leading zeroes to the number if you have one.
Now seem to have a slightly shaky grasp of what form addresses can take, and also to be using different validations for adding them, from those used when changing them, so you can put them in, but then get stuck trying to change them 😢
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.