Translation:Now, everyone, let's stand up.
But in everyone stand up, you can interpret stand up as being in the subjunctive mood, therefore making the clause a command.
It is not apparent here because stand has the same form whether as indicative or subjunctive, but for another verb such as be, it would be explicit: “Everyone be there at noon!”.
PS: actually, the subjunctive is apparent even in the first sentence, since everyone is considered as using third-person singular; the statement would have been “everyone stands up”, which MattDailisan did not write — and thus, he is correct.
Sometimes Duo does very tricky stuff with their tiles like breaking words up into syllables and adding the last syllable of one word onto the first syllable of the next so it is actually possible to have a typo when using the tiles - worth having a look over your answer again at least and then if there's definitely no error report it
It s a bit strange that Alright everyone, let s stand up is not accepted. Also, you ll notice that people often use this pattern...さあ、みなさん、何々しましょう...when trying to get small children to do their bidding. In that sense it can be a gentle, but imperative, instruction to the kids rather than an overture to really do something together.