One or more elements cannot be resolved during compilation because the element is not defined by the HTML spec, or there is no component or directive with such element selector.
HELPFUL: This is the compiler equivalent of a common runtime error NG0304: '${tagName}' is not a known element: ....
Debugging the error
Use the element name in the error to find the file(s) where the element is being used.
Check that the name and selector are correct.
Make sure that the component is correctly imported inside your NgModule or standalone component, by checking its presence in the imports field. If the component is declared in an NgModule (meaning that it is not standalone) make sure that it is exported correctly from it, by checking its presence in the exports field.
When using custom elements or web components, ensure that you add CUSTOM_ELEMENTS_SCHEMA to the application module.
If this does not resolve the error, check the imported libraries for any recent changes to the exports and properties you are using, and restart your server.