Angular 6 Library - Could not resolve entry

I was facing the same issue. My mistake was:

In one of my service in the library, I used environment variable and imported it like this:

import { environment } from 'src/environments/environment';

So while building, it couldn't get the relative path and was showing the error. I removed it and it worked fine.

Please ensure that in your library module, you are not referencing any main project related item or not importing any relative path