Can't login to my google account after factory reset.

My sister forgot her lock screen pattern, so I thought factory reset would do the trick.
After the reset, it asks me to sign in to a google account that was previously synchronized with the phone. I put all the information, and it tells me that there's "an unknown error. Please try against in 24 hours". I waited for 24 hours, and the same message appears.
I have tried every single option that was given by everyone with similar problems, and none of them work.
I tried signing to the google account on pc and removing the device, but no such options exist.

The phone my sister is using: Just 5 Cosmo L808.
I am out of ideas, and I really need help so my sister could have a phone again.
Thank you.

Are you putting in the Google account that she had in the phone before you reset it?