So I assume your Saving works fine in Editor and in android api<29
but not in android 10-11-12 so here is how I solved It
===Step 1===
In Project settings>Android>Advanced Apk Packaging
Extra Premissions. Add those 3 Elements:
android.permission.READ_EXTERNAL_STORAGE
android.permission.WRITE_EXTERNAL_STORAGE
android.permission.READ_INTERNAL_STORAGE
android.permission.WRITE_INTERNAL_STORAGE
===Step 2===
In Project settings>Android
set “Use External File Dir for ue4 Game Files” to True
===Step 3===
in Project settings>Packaging
set “Build configuration” = Shipping
Set “for Distribution” To False
===Step 4===
Package the project but Don’t Test Yet
===Step 5===
Generate the Signing key USING ANDROID STUDIO!
-watch this tutorial to learn how:
-How to sign your Android application properly for Play store in Unreal engine 4 - YouTube
-Then Fill the Data in Android>Distribution Signing
===Step 6===
in Project settings>Packaging
Set “for Distribution” To True
Now Package the Project and Test the Game
you might need to Turn off the Internet before Installing the Game
But you won’t have to do this after you publish it to Google play
you’re welcome. 