I remember picking up the quest at the LAST brother in the ruins. It didn't appear until after I turning in one of the other quests to him though (can't remember which one)
If it doesn't work on multiple browsers then you are almost certainly doing something wrong. Are you sure the email isn't already in use? The errors on the website aren't always accurate, I think.
Does anyone has the same issue? I recently created a new character, I cleared all quests of Red Ribbon HQ ruins but I can't find where to accept the quest of defeating Android 9(final boss).