blitzou Check the post by espresso8031 right above yours. I had the exact same error and all I did was uninstall MW3 and Plutonium, then redid the downgrade process except with those depot files.

CannyExpert9 Banged my head against the wall for a couple days at this too, what worked for me was uninstalling MW3 and Plutonium completely then redoing the downgrade except with the depot files found here: https://forum.plutonium.pw/post/178617

Literally just learned about Plutonium and was wondering why I couldn't get it to work on Linux even following the linux guides and noticed there was a lot less stuff in the folder than there should be, including missing mss32.dll. Now I know why. If this literally just happened 3 days ago this probably sets a new record for worst possible timing lol