Cordova diagno Installation failed

My ionic- info

cli packages: (C:\Users\Admin\AppData\Roaming\npm\node_modules)

@ionic/cli-utils  : 1.19.2
ionic (Ionic CLI) : 3.20.0

global packages:

cordova (Cordova CLI) : 8.0.0
Gulp CLI              : CLI version 3.9.1 Local version 3.9.1

local packages:

Cordova Platforms : android 6.2.3
Ionic Framework   : ionic1 1.1.0

System:

Android SDK Tools : 26.1.1
Node              : v6.11.0
npm               : 5.6.0
OS                : Windows 7

Environment Variables:

ANDROID_HOME : D:\Android

Misc:

backend : pro

Can’t able to add cordova plugin add cordova.plugins.diagnostic@3.2.1.
It shows Failed to fetch plugin cordova.plugins.diagnostic@3.2.1