I had a similar problem, after initially setting up pycharm with one of our projects to run it locally in a dev envrionment setup. Therefore i was very puzzled to see that it did not find my local files but resolved all the other imports. And it was not easy to find the right question and answer based only on the error message.
But yes, exactly the selecting and marking of the directory "src" as "Sources root" helped.
I would have expected that this is default. ;-)