My guess is that one of the mirrors was/is down (or not up to date). I just double checked on the mirror and it's definitely there (at least it is on the mirror I was redirected to). I also tested from the Magento page and that worked ok for me too.

So I'm pretty sure you must have a hit a "bad" mirror. Perhaps try connecting directly to the "master" mirror directly and see if that works. http://mirror.turnkeylinux.org/turnkeylinux/images/ova/turnkey-magento-15.4-stretch-amd64.ova. (Please note that the "master" mirror, isn't actually a single mirror, it's a few high bandwidth mirrors with "round robin" DNS - so if the issue is with one of those, then it may still occur).

Hopefully it was just a "one off" issue, but if it happens again or you're getting the issue consistently, please let me know and I'll investigate a little deeper.

Also, there are direct links to specific mirrors on the mirrors page.