Can't find 'apksigner' executable to manually sign APK

apksigner shipped with Android SDK Build Tools 24.0.3 (see https://developer.android.com/studio/releases/build-tools.html). The tool can be found in the Android SDK's build-tools/<tools version> directory. For example, build-tools/24.0.3/apksigner on Linux/OSX and build-tools/24.0.3/apksigner.bat on Windows.

On mac
The Apksigner is inside build tools all you need is this line

/Users/adewale***/Library/Android/sdk/build-tools/28.0.3/apksigner verify MyApp.apk

just change /Users/adewale***/Library/Android/sdk/build-tools to your build tools path. Done

On Windows 10 it should be under directory

 C:\Users\{user}\AppData\Local\Android\Sdk\build-tools\

Note the AppData directory is by default hidden. First make sure you installed Android SDK 24.0.3 or later.