You can see the post here:

https://community.amplitude-studios.com/forums/85-help/threads/51538-dungeon-of-the-endless-out-of-stock-error-occured-can-t-redeem-on-steam?page=6#post-368034

I'm just calling it out in a separate thread so that I can track it and it's not lost.  The problem is that your site thinks it activated the game, but Steam thinks it didn't.

FYI, this is still an issue

still can't claim it too..