Have you tried creating a binary file and writing all save related data to it? The file should be loaded at the startup of a game.
I don’t use blueprints but you can create your custom blueprint node that will do that, check out this answer How do I create my own blueprint node with C++? - Blueprint - Epic Developer Community Forums
If the highscores are only local that would be enough, but if you are uploading them to online highscore table you should include some kind of encryption to prevent people from altering their highscores by messing around with the save file.
Also make sure your app has the permission to read/write files
add <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" /> to Android manifest file
Hope it helps