We are aware that the Windows 10 code redemption portal is giving an error again, but we'll have more codes for those of you attempting to redeem before the April 20th promotion end date.https://www.minecraft.net/en-us/article/ancient-java-players--you-attention-please- …
This is most likely a temporary issue, but please try logging out and into your Microsoft (Xbox Live) Account in the Microsoft Store first. I hope this helps! ^SA