So, this apparently has been an issue for a while now (look back in the forum, it goes back at least a year). Tried to set up auto-pay, and it failed. I haven't used the card thank god, because as I searched for a solution instead I found horror stories of being unable to pay credit card bills because of this same error!
How frustrating. People call Synchrony bank and wait for half an hour, agent's either don't understand the problem or aren't helpful. If they do manage to pay the bill it's $10 to pay it over the phone... like, really? Holy crap guys.
And before you ask... Yes I've cleared cache and cookies. Yes I've tried Chrome, Firefox, Edge, and tried incognito mode. Yes I've doublechecked all banking info. Yes I've tried on mobile. And no, it still isn't solved. So save your breath if you're going to suggest any of that. If I were a technical support rep who is browsing this forum, I'd suggest opening an internal ticket with Synchrony bank about this issue.