1. If 'ion-content' is an Angular component, then verify that it is part of this module. 2. If 'ion-content' is a Web Component then add 'CUSTOM_ELEMENTS_SCHEMA' to the '@NgModule.schemas' of this component to suppress this message. code example

Example: If 'ion-item' is an Angular component, then verify that it is part of this module.

I had this problem when I was importing the page (component) rather than the module, and it wasn’t giving any compilation errors but then module not known at runtime. In your app.module.ts put your module in the imports list.