Wouldn't a better translation be 'they are sending a letter to Japan'?
Or sent a letter
Saya mengirim... = I send... = I'm sending... = I sent..., in Indonesian we don't recognize tenses.
Maybe Duo wants to teach us something because there's slightly different in tenses. For example: I'm sending... = Saya sedang mengirim...
So "the letter" is wrong, but "a letter" is correct. Not sure why?
It should be accepted. Indonesian doesn't burden "the" or "a/an" much.
Saya mengirim surat = I send the letter = I send a letter
Saya mengirim surat itu = I send the letter = I send that letter
Saya mengirim sebuah surat = I send a letter.