I have come across lot of threads on gamers and Prototypers expressing their frustation over Save slot issue, with none of them providing any Feasible Solution.

You have landed on the right page and this is the only Guide to provide 100% Workable solution.
No, The developers of the game have nothing to do with it. This game was launched in 2012 and none of them foresee a future possibility of windows 10 and an apocalyptic os windows 11.

Anyways, So I believe you are able to launch the game and it displays a warning sign of something like "Failed To Save Data" and in-game saves are crashing and when you re-launch the game, it simply force you to start over. Worry not, just follow this step-by-step guide (And no, anyone saying Run as administrator by going to local files folder and tweaking .exe file properties isn't gonna work).

First things first, you are using Windows 10. Great. Let's proceed !!! (Sorry dear Linux or mac users)

Method 1:

Uninstall Windows 10,clean your hard drive and ask your friend if they still possess a setup of windows 7 with them and re-install prototype 2 on good ol win 7

Well, Of course, this is a whole new level of madness and at this point , you might want to jab my nuts BUT its just one of the 100% fix aligned with my guide's title......NOT HAPPY !! Anyways, so, let's discuss method 2.



Method 2:

I'll try to be articulate as I can. Basically, you need to switch the user on your windows 10/11 with a local one. So, let's do it. Just follow the instructions.

1. Grab a pringles or a pack of your fav snack cuz it might take some time.
   
2. Sign out of your one drive. Don't uninstall, just sign out until you complete the game.
   
3. From the start of windows, Go to settings->account.
   
4. From there, go to Family & other users.
   
5. Under Other Users panel, click on "Add Someone else to this PC".
   
6. Select I don't have this person's sign-in information.
   
7. Now, Add a user without a microsoft account. (Any user, just create one).
   
8. Add user name and password etc.
   
9. After creation, From the same page, Click on user icon-> Change Account Type.
   
10. Account Type->Administrator. (Make sure you select account type as administrator)
    
11. Now go to start icon. From there, see the left hand pane, where there are icons like switch off, settings, images, etc.
    
12. From there see the icon with user badge, Click it.
    
13. Select the newly created user name you just added. Switch to that user now.
    
14. Now log in to a new account.
    
15. Now uninstall and re-install or fresh install Prototype 2 from there.
    
16. That's it ! Play the game from this new account now. It will run without any annoying save game issue.

If there is any method 3 , that's simple, Play the game in one-go which is even worse than method one :-D

This game was far ahead of its time when it got released. I was 12 or 13 back then and never really played a lot of games until I grow old. I am pretty sure the windows will keep on upgrading their OS'es and with every new upgrade, old apps and features designed to run in previous versions lose their ability to run in newer ones. That's not gonna keep us geniuses to give up and abandon old titles altogether. Any newcomer (just like me) or someone who played this already who just want to come back in this contagious open-world based game, this guide will serve as a reference at present or in future.

Hope this guide was helpful. Do let me know under comments if you need any help. This is my first-ever steam guide. Well, just letting you know