If 'ion-button' is an Angular component, then verify that it is part of this module. 2. If 'ion-button' is a Web Component then add 'CUSTOM_ELEMENTS_SCHEMA' to the '@NgModule.schemas' of this component to suppress this message. #6
If you get this error on any ion declaration you need to check the module of the component and add it to the module declarations:
@NgModule({ ... declarations: [MyComponent] }
Try:
Before :<ion-button></ion-button>, After :<button ion-button></button>
It worked for me.