This thread has run its course and would be locked. Those seeking Minecraft Windows 10 edition should see a code on the redeem page.
Windows 10 Code Error - Fixed!
https://help.minecraft.net/hc/en-us/articles/360038403371-Windows-10-Code-Error
The redemption program would end on April 20, 2020 so make sure to get the code before.
If you purchased Minecraft Java Edition before October 18, 2018, you will see a button that says “Redeem” in your account.
· If the button is grayed out, you’ve already claimed the code. Log in to your Microsoft account to redownload the game.
· If you don’t have a Redeem button, you purchased Minecraft Java Edition after October 18, 2018 and are not eligible for this promotion.
update: 17th April 2020
Mojang are aware that there are no codes available at the moment, for those eligible. They are actively working on this and will get it working before the deadline.
Update: 18th April 2020
Mojang have posted that this problem with the codes has been resolved, and you should see the option to get your code if you are eligible. Please try redeeming your code now.
Update: 21st April 2020
Offer has now ended and you will be unable to redeem your Windows 10 Minecraft code now.
More information can be found on this Twitter post: https://twitter.com/MojangSupport/status/1250820862975963136