Yes, they're both accepted everywhere through-out the course, though i går is the default spelling. If Duolingo showed that as an error to you, and it wasn't a "type what you hear" exercise (because those are buggy), it should be reported.
Ah, you got a typo notice. If it was a "type what you hear" exercise, then that can't be changed because of how Duolingo works. Otherwise, it's, well... also a bug. :p But I assure you, they're both accepted everywhere.