Firstly are you using an older nowtv white box which doesn't support Sky Sports Box Office anymore.
Or if you are using any of the other nowtv boxes or stick, then if this is the case i don't think the App is currently live because there is no events currently available to buy or watch, where i believe the next event is the Joshua 2 fight during early December.
If you haven't used Sky Sports Box Office recently you may have to re register you details again because the Sky Box Office App was updated (have a read of this link page with the FAQ about Sky Sports Box Office where there is a section on "Why i need to Re-register" ).
Also see if anything in this linked article helps with your question ?
Firstly have you updated your account details since the 5th of August 2019 ?
i have Copied & Pasted this information below from the Sky Sports Box Office FAQ.
Thanks again, I knew about having to register again, & have done so as far as I'm aware. Tried the forgotten password but to no avail. I receive no email to change it. Appreciate your help & will try again at the weekend, many thanks.