Simple example

Wrong import { BehaviorSubject } from 'rxjs/';

Correct import { BehaviorSubject } from 'rxjs';

I faced this issue during a migration. Search for something similar in your code to fix the issue. In my case the component causing the issue was mentioned in the error message.

Check your imports, you may find a path that ends with a '/', just remove it and it will work!

import { HttpClientModule } from '@angular/common/http/';

just remove / from last of your import as Thomas Renger mentioned above.

like this

import { HttpClientModule } from '@angular/common/http';