@pocketfIower Are you sure that Sims 4 is still using the OneDrive folder? It's been known to switch, and if you end up putting a save in the wrong location, it won't show up. The easiest way to check is to launch the game, click Load Game, and hover over the folder icon under the info for one of your saves. That will show where the save is located, or specifically where the copy the game is reading is located.
At any rate, if you open a File Explorer window, click This PC, and search for Slot_0000000b (the exact name of the missing file in your screenshot), you should be able to right-click and copy the file, and then you can paste it onto your desktop for safekeeping. Please make sure that it has a .save extension and that it's not a trivial size that would suggest its data is missing. Then you can click on the file and Open file location (the option is at the top of the File Explorer window), and the file itself should be highlighted in the window that opens.
You can also do the same search for Slot_* (with the asterisk), again in a File Explorer window rather than in the search bar at the bottom of your screen, so you have the extra options. You might have to wade through a number of files, but sorting by date should help.