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:
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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).
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Open Modern Warfare.
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Go to the Multi-player section.
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Install the multi-player packs. If you've already installed them using the other account then this part only takes a few seconds.
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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.
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Select 'Manage Game Content' also in the options menu.
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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.
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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.