Well, there are many reasons why but I will show you most common errors. Note, i'm talking about general apk modding, not system apk/framework modding Assets.dex issue "Class xxx has already been interned" and "Not a valid dex magic value" By default, apktool try to decompile dex from assets folder which would decompile to smali_assets on a root directory of decompiled apk, and compiled to assets.dex upon compiling This can cause compiling issues Solution: To fix the problem, decompile APK again with the flag [code]--only-main-classes[/code] to only decompile on main classes in root (classes[0-9].dex) then you can compile without any problem Note: Only for apktool 2.4.1 and up You can try to delete .dex from assets folder or smali_assets/assets.dex but it may cause problems If you are using APK Easy Tool 1.57 and above, you can enable the option "Only disassemble the main dex classes" under Options -> Apktool. Aga...