Can't install .apk after decoding and rebuild (Apktool, Apkstudio)

Non-system apks need to be signed after recompiling. Try to sign your apk: Signing Your App Manually

I Know it was answered before but for more clarification , you can sign it by these commands.

First Generate the Key :

keytool -genkey -v -keystore my-release-key.keystore -alias alias_name -keyalg RSA -keysize 2048 -validity 10000

Second sign it :

jarsigner -verbose -sigalg SHA1withRSA -digestalg SHA1 -keystore my-release-key.keystore my_application.apk alias_name

IF you Don't like to enter a password after previous command or do it in bash file, use this :

jarsigner -verbose -sigalg SHA1withRSA -digestalg SHA1 -keystore [Your Key Store] [Address/YourApk.apk] alias_name  -storepass [Your Password]