The official PvZ3 is still on your phone, or you have a conflicting signature. Fix: Go to Settings > Apps > search for "Plants vs Zombies" – even old versions. Delete them all. Restart your phone. Try again.
Verify that the OBB folder is placed exactly inside Internal Storage/Android/obb/ and matches the package name required by the mod developer. Game Asks for an Update plants vs zombies 3 new start mod install