I added ionic to an existing angular application and ran into the following troubles after adding the ionic core css:

Error: Failed to find ‘@ionic/angular/css/core.css

Solution

When importing the css do this:

@import '@ionic/core/css/core.css';

instead of

@import '~@ionic/core/css/core.css';

Postcss cannot handle this tilde, but will interpret it correctly without!

A possible other solution could be that your IonicModule is not imported well. Add this to your app.module.ts imports []:

IonicModule.forRoot({
  animated: false,
}),