Try using Revo Uninstaller ( the free version is absolutely fine! it does the same job ) to uninstall the EA app. Then, using a fresh app installer file, install the EA app again. Log in as yourself and The Sims 4 game will be in your library but not playable. You will have to download it again. Once you have downloaded it again, try redeeming the code again. The Sims 4 will be 'new' to your library and so there's no reason why it should say you don't have the base game! None of your previous game progress should be lost unless your console/pc does something you are unaware of during the uninstalling, installing and downloading. Hope this helps, my love xx
I DON'T WORK FOR EA BUT I LIKE TO HELP PEOPLE FIX THEIR GAMES! 🙂