Duo used "recordar" for "remind" in this very section.
Check it out: https://forum.duolingo.com/comment/480127
According to the comments, whether it means "remind" or "remember" has something to do with whether it's in the indicative or the subjunctive, but I'm not even going to pretend I have a real grasp on that.
That's for indirect object nouns, but generally not for direct object nouns:
Él vio a su vecino. He saw his neighbor. (direct object)
Él le escribió a su vecino. He wrote (to) his neighbor. (indirect object)
You might be thinking about optional a+pronoun phrases:
Él lo vio (a él). He saw him. The "lo" is required, but the "a él" is optional if you need to clarify that it's not "He saw it" or "He saw you (usted)."
Él le escribió (a él). He wrote (to) him. The "le" is required, but the "a él" is optional if you need to clarify that it's not "He wrote her" or "He wrote you (usted)."
So in duo's sentence, you don't use a "lo" because you have a direct object noun (vecino) instead of a direct object pronoun.
In a similar sentence the "personal a" before the Spanish words "su amigo" was not needed and marked wrong. Now it is acceptable? Neighbors are more personal than friends? I love Duolingo but the lack of consistency can sometimes make this more frustrating than it has to be. I'll quit crying now and keep practicing ;-)