I received a $3,000 voucher on December 2. Is it valid?Can you buy it?

You have to wait until Blockstack deems it a “non-security.” So hopefully sometime before the network launch in 2019.

sorry, what does this mean?

if I scan the QR code on the voucher, it says sale has ended. Is that correct?

If yes, what is point of sending voucher so late.

Edit: I think I got it. It is just a voucher, which enables to buy tokens worth USD 3000 (at 0.12 per token).

This is correct. You’ll get a follow-up email from Blockstack sometime in the future when you’re able to buy the tokens at the ICO price. But this will only happen if it’s deemed a non-security by their legal team.