This happened to me with the previous patch (1.10), and the issue I had was that the registry no longer had the correct version and build. I had to alter it from BUILD 28 to 40 (and VER to 2.0.0.40). I'm almost certain that that's the same thing stopping me from installing this current patch, but I'm not sure what build of the game we're in, nor where to find that information. Thanks in advance for any help you guys can give me.

You don't update your game by fiddling in the registry. The registry entry is just a flag for the *actual* update process. If the installer is telling you that you have the wrong version of the game, then you have the wrong version of the game. Download the 1.10 installer, re-install and then apply the patch. Editing the registry it totally unnecessary, and highly likely part of your problem. Don't do it.

I already did all of that before posting here. This is on a fresh install of the game, and I'm still getting the error. That is why I suspect the registry build/ver number could be the cause. Editing the registry wasn't my go-to either, I've only done that once with this game, and the reason for that is in the OP. After uninstalling, re-installing, and verifying the game files I checked the registry to see if it changed, but it still says build/ver 40. Now I'm not sure if that's what it's supposed to be (maybe someone else here with patch 1.12 installed can verify what their current build/ver number is), but thus far it doesn't seem like it.

Turns out the build/ver in my registry wasn't up to date again. Changed the BUILD to 43 (VER 2.0.0.43), and the 1.12 patch installed without any issues. Game runs fine.

Thanks og_moogle. Your BUILD and VER value worked for me. I installed Witcher 3 on an external hard drive and couldn't install the update because I had reinstalled Windows.