"I did not remember it."
Translation:Non lo ricordavo.
In English, "remember" can mean both "have in memory" and "recall/recollect". As far as I know, in Italian, the imperfect of "ricordare" means the former, while the passato prossimo means the latter. Maybe Duolingo is only taking "remember" to mean "have in memory"? Given that this is a lesson on the imperfect, it would make sense to interpret in the imperfect sense and only accept imperfect conjugations.
It isn't correct, as it doesn't account for the original "it". Duo sometimes gives bad corrections, because it seems that only one error at a time is changed. If you'd said what you tried ...
Ricordare and ricordarsi are a nuisance. They seem to be partly interchangeable, but any rules for when you can and can't use either are not clear to me. However, if you use the intransitive ricordarsi with an object you do have to add a preposition: di [oggetto]. And when your object is a pronoun (here "it" = lo) you have to merge it with di into the particle ne. Hence I just had non me ne ricordavo accepted; boy was I surprised!
if you used 'ricordarsi' instead of 'ricordare' you could say "non me ne ricordavo". ridordarsi is reflexive and in English would be equivalent to saying 'to remind oneself of/about something'. "I did not remember the time"="io non mi ricordavo del tempo". you could replace 'del tempo' with 'ne'. the 'of something would be the 'ne' (me ne ricordavo). but you could also say 'me lo ricordavo'. https://www.thoughtco.com/using-ne-in-italian-4074179