We literally say Nice to meet. The verb träffas is a deponent verb, read more about those here: https://www.duolingo.com/comment/6094592 Anyway the meaning of träffas is reciprocal, meaning that two or more people do something to each other, which makes it a little easier to leave out the object, since the verb in itself presupposes that two or more people are involved.
Edit: let me rephrase that. It's not just that you can leave out dig/er, in fact you can't use it with träffas. You'd have to put some preposition in between for that to work. This is because, as I said, the verb already in itself presupposes two or more people acting on each other.
I think träffas is kind of intransitive verb and it can't have an object after it, as it should be translated to meat each other. Like in sentence:
Ve träffas utanför dörren.
I think this sentence is better understood as "it is nice that we meet each other", but this is not how one would say it in English.
It is the same with verb ses/hörs. It would be vi ses/hörs without any object after them.
"Good to meet you" was marked incorrect. Is there a better translation for that? I'd use the two almost interchangeably, but I can appreciate the nuance.
I ended up trying a lot of variants; street is also not accepted. That may be a regional thing? Where I grew up, the two are pretty much interchangeable. Street sounds more urban, but either could be used without sounding odd.
Duo really makes me appreciate nuances in my own language that I'm not even aware of!