I was facing the same problem, found a solution
a) uninstall platform of android
b) uninstall cordova plugin background mode.
c)
npm uninstall --save @ionic-native/background-mode@4
and or ```
npm install --save @ionic-native/background-mode
then refer https://ionicframework.com/docs/v3/native/background-mode/ to install
$ ionic cordova plugin add cordova-plugin-background-mode
$ npm install --save @ionic-native/background-mode@4