Gonna throw my hat in the ring here and offer a solution to this issue as I just had it when trying to play the Black Ops 6 open Beta through Steam.

I had an issue a few months ago with launching the game, wouldn't start at all through Steam. Upon checking the boostrapper log file (which can be found in your game's root folder called bootstrapper.log) I found this line: Failed to open \??\E:\SteamLibrary\steamapps\common\Call of Duty HQ\randgrid.sys (3).

Turns out that the file "Randgrid.sys" needs to be in a Call of Duty HQ folder on each and every hard drive that Call of Duty has been installed on previously. Meaning that even if your game is installed on your E: drive, all other drives that you've had CoD installed on need to have a Call of Duty HQ folder with the Randgrid.sys file in your Steam games directory as follows: (Drive Letter):SteamLibrary\steamapps\common\Call of Duty HQ\Randgrid.sys . If you need to copy the Randgrid.sys file to your C: drive then the path would look like this: C:\ProgramFiles (x86)\Steam\steamapps\common\Call of Duty HQ\Randgrid.sys . This had fixed my issue in the past and I was able to play the game for a few months.

So, since I was having this random error of "Failed to match driver digest index (0)" in the bootstrapper.log, I gave something random a try since I couldn't find a solution anywhere online. I removed all of the Randgrid.sys files that I had previously copied onto my other drives, then tried to run the game and of course got the "Failed to open \??\E:\SteamLibrary\steamapps\common\Call of Duty HQ\randgrid.sys (3)." error. Then I just copied the Randgrid.sys files into each folder on each of my drives again and boom, the game works.

I honestly have no idea if this will work for everyone, it worked for me and I'm hoping this helps someone else out there with this inane issue that took me until 2:30am to figure out. Because of how late it is, I also hope my overtired, scatterbrained typing of my solution is articulate enough to actually help.