Eric, the 3.20.5 version of HPLIP you are installing is most probably not for Ubuntu 20.04. Ubuntu 20.04 has Python 2 completely removed and only uses Python3. For Python3 there is a python3-pyqt5 package available, and this is used by the HPLIP which comes with Ubuntu 20.04 (the hplip-gui package).
Eric, the 3.20.5 version of HPLIP you are installing is most probably not for Ubuntu 20.04. Ubuntu 20.04 has Python 2 completely removed and only uses Python3. For Python3 there is a python3-pyqt5 package available, and this is used by the HPLIP which comes with Ubuntu 20.04 (the hplip-gui package).