You're not the only one - it's happened to me on 2 separate devices that were both working perfectly fine earlier today.
Looking for a possible solution, found lots of other disgruntled people it has also happened to over the past few months, many of them posting pleas for help in this very community only to get VM "support" give them the usual help desk fob off questions (are you sure you haven't got caps locked on/off, have you tried rebooting device, uninstall and reinstall app, etc) - none of which works, so after a few days faffing about they raise a ticket with "technical support" (who are you then?) which means you get put on hold for another 5-10 days before the next person in the food chain bothers to have a look, scratch their head and then pass you on to some other department (if you've ever had problems with any part of your VM package before you'll know the drill).
Anyway, it looks to me to be a fault at the user validation server end. Put in a wrong user ID or password and it will chuck you out with a sensible message, try the real ones and you get that "Oops" message about needing to be your main account not a sub-account (even though you know you're using the right one - possible because you only have your main one and no sub's).
I tried clicking on Register which then asks for your account e-mail or mobile number. Either one I give comes back with a hit - you are already registered and here are the accounts associated ...1 sub-account that I set up for my wife more than 10 years ago that has never been used and a null field! No sign of my actual user ID.
The same login works fine with all the other VM apps and the website so must be specific to just TV Go, and not an isolated thing for so many other people to have hit it over so long a time - which you would think would mean someone with a bit of nous would have spotted it, identified the issue and, if not source fixed it, made the workaround generally known and available so the community support wallahs on here and in the call centres could sort you out immediately.
Let's see how long it is before our cries for help are answered.