To install push you must first install cocoapods. Follow these steps on your terminal in the Ionic project directory.
First remove what you tried to install
ionic cordova plugin remove phonegap-plugin-push
Next install cocoapods
sudo gem install cocoapods
Then you need to sync the cocoapods repo
pod setup
This may take a while
Finally install push
ionic cordova plugin add phonegap-plugin-push --variable SENDER_ID="XXXXXXX"