So, Meno ends with Meno and Socrates agreeing that virtue cannot be taught. But there is reason to doubt whether Plato intended the reader to draw this conclusion. There are two arguments in favor of virtue being knowledge (and so teachable), and two arguments against virtue being knowledge (and so teachable). The arguments against are conspicuously weak (though the weaknesses are, admittedly, not highlighted and discussed in the text), and the arguments in favor are never refuted, just set aside.
More notably, earlier in the dialogue Meno said "I have made many speeches about virtue before large audiences on a thousand occasions, very good speeches as I thought" (80b), and Socrates references this during the slave boy discussion, commenting on how the slave boy could no doubt confidently have made many speeches about how to double a square. And another thing in the background (and suggested by some of Meno's uninspiring answers) is Meno's own conspicuous lack of virtue. So my preferred interpretation of the dialogue is that it ends the way it does because it would be a good thing for Meno to stop making speeches about virtue; teachable or not, it is not a subject Meno should teach about. If he is convinced that it can't be taught, maybe he won't make any more unfortunate speeches before large audiences on this subject where he is clearly extremely badly informed.
In Protagoras, Protagoras and Socrates discuss both whether virtue is teachable and whether it has parts, with Protagoras arguing for both claims and Socrates arguing against. At the end of the discussion, Socrates summarizes where they have ended up by suggesting that Protagoras seems to have the upper hand in the first debate, but Socrates thinks his own arguments were more convincing on the second. Cooper snarkily suggests that this has still established that nobody should go to learn virtue from Protagoras, as he doesn't have a coherent account of what it is. But in light of how the ending of Meno worked, I wonder if we should be so hasty; if Socrates is leaving the possibility of virtue being teachable on the table, as he did not do in Meno, is there perhaps an implication that having Protagoras try to teach virtue is at least not so horrible a prospect as having Meno try to teach it? Protagoras is also mentioned in Meno, of course, and in Meno it is Anytus (surely not a reliable source!) who insists Protagoras is no teacher of virtue, while Socrates seems inclined to defend him. That there is that mention of Protagoras in Meno also contributes to my suspicion that Plato may have intended readers to make connections along the lines of what I'm suggesting.