To start, I've tried literally everything and watched so many videos with no success in fixing this error. I can't tell you how many times I deleted and reinstalled everything.
Note: My specific error had all add ins installed within the game, but showed others not installed under 'Manage Game Content'. When I would install those add ins they would revert back to not installed once the game was opened again. In game add ins would still show installed. I am also using the disc edition of the game.
I finally found this video, and this fixed it for me. I can't guarantee that it works for everyone, but it was a fix I never saw before:
https://youtu.be/BVHrJlAk-bI?si=Xjg4_OhYDqSa0iJD
The quick and dirty of what I did that worked:
Switch from the affected account to another PSN account (literally any other account, it doesn't have to be the one that originally installed the game).
Open Modern Warfare.
Go to the Multi-player section.
Install the multi-player packs. If you've already installed them using the other account then this part only takes a few seconds.
Back out to the main screen of the PS5 and move to the Modern Warfare tile, then click the options button and select to close Modern Warfare.
Select 'Manage Game Content' also in the options menu.
Install any add ins here that aren't showing installed. This will probably also go fast if you've installed them on the other profile as well. These only took a few seconds for me.
Once those are installed, switch back to your account and open Modern Warfare and see if the error is still showing.
I have no other steps after this if it's still not working, but these worked for me. I would suspect the steps are similar on PS4. I don't have an Xbox, so I don't know how well the steps would transfer over.