flask again

For reasons unrelated to pga4, I had to reinstall it in a new venv. I am now on Python 3.6.3 and Ubuntu 16.04, and I am working with pga4.2.0, which is a newer version than I had before, and which worked with Python 3.5. I am getting the import error related to flask that I have seen many other posts about, but none of the solutions work for me.

Flask and PGA4 are side by side in the same site-packages. They were all installed at the same time, as was libpq-dev and python-dev   https://stackoverflow.com/questions/41260004/error-trying-to-run-pgadmin4

They are in the same venv: http://www.postgresql-archive.org/pgadmin4-1-6-python-wheel-doesn-t-work-under-Ubuntu-Server-16-04-LTS-td5971865.html

I have seen no errors related to "yarn" or "qt": http://www.postgresql-archive.org/v2-0-rc1-not-starting-runtime-not-building-td5982724.html

What is my solution? Thank you.