"You got two messages" was the only sentence suggested that was grammatically correct. It was marked right. "Got" is a terrible English word, and not really appropriate for "пришло," in my opinion. "Got" works in English, I suppose, but would translate better as "received" - получил. Пришло should be "arrived."
Purists might dislike ' got ', but it's perfectly grammatical and widely used. What's more I'd far rather use an honest Anglo-Saxon monosyllable like ' got ' than a mealy-mouthed Latin obfuscation like ' recieved '. ' I got your message. ' ' I am in receipt of your missive. '
? "You got" is past tense in standard English. Colloquially in North America it is also present tense possession, but that's a linguistic register below where Duolingo trods (although it's extremely widely used).
"You have got" / "You've got" is for all intents and purposes present tense possession in North America, although I think it still maintains its etymological present perfect meaning in the UK.
I think there is something odd going on here - the apparent subject of the sentence is два сообщения, which is a plural, even though сообщения = "messages" is genitive singular.
The fact that пришло is neuter singular indicates to me that there is a "dummy" and omitted subject in this sentence: "it" or "there" = "It/There arrived for you two messages."
If, as Alexmalaho say, you can use either Пришло or Пришли, changing the verb form would indicate a change in the subject: the subject of the plural verb would be "two messages", and there would be no dummy, omitted subject.
Trying to figure this sentence out in terms of the rules for structuring numbers of things in Russian just doesn't work. The subject for Duo's sentence has to be some dummy subject, "It has arrived for you two messages".
Using a plural verb is much more direct and makes more sense from an English-speaking point of view, but I've looked at other sources, and the singular verb Пришло seems to be an idiom which may be preferred, even if the plural verb Пришли would be accepted and easily understood.
"Пришло" is the verb "прийти" in the past tense, singular, neuter. As Alexmalaho said, in this case both singular neuter or plural form ("пришли") can be used. The neuter form is possible because technically in Russian the subject of the sentence is "two" not "messages". "Two" is a numeral, not a noun. When the subject is not a noun, it's usually treated as if it was a singular neuter noun. However from a different point of view the focus is on the messages and there are more than one, so using the plural form is also correct. It's mostly a question of preference.
In Russian, the past tense form of the verb changes depending on the gender & number of the subject noun. So: he came = он пришёл (masculine, singular subject) // she came = она пришла (feminine, singular subject) // it came = оно пришло (neuter, singular subject) // they came = они пришли (plural subject)