ionic geolocation plugin error: "No provider for Geolocation"

You need to import the Geolocation class and add it to your list of providers in the app.module.ts file

import { Geolocation } from '@ionic-native/geolocation';

providers: [
     Geolocation
]

You need to add the provider to the NgModule, i.e module.ts under providers,

providers: [
  Geolocation
]

For Ionic 4 you add to app.module.ts top:

import { Geolocation } from '@ionic-native/geolocation/ngx';

And on the bottom, inside of the Providers array, add Geolocation:

providers: [
    Geolocation,
    ...
]