Your Anti-Virus could be deleting the file. Yesterday you might have flagged it as an exception but on startup it was still deleted. Disable your Anti-Virus then download it and add it as a permanent exception on your Anti-Virus, also allow it threw your Firewall.
Unturned has a specific help section that you should have posted this in.
https://www.mpgh.net/forum/forumdisplay.php?f=806