Prerequisites

The plugin connects itself with the PostgreSQL database through Python's QtSQL package. In most environments running QGIS all required packages will be installed anyways.

If you get an error like ImportError: cannot import name QtSql you have to manually install something like python-qt5-sql (Ubuntu) or python-qt5-utils (OpenSuse).

Additionally you need Qt's PostgreSQL driver. If you get the error Driver not found, you should install it manually, search for e.g. libqt5-sql-psql (Ubuntu).

If you installed QGIS on Windows from the standalone installer or from OSGeoW4, the required packags should have been installed.