Note that there is a new QGIS installer, described as Standalone installers (MSI) from OSGeo4W packages (recommended for new users). This also has a long term release version 3.16. It uses a different version of Python (3.9), from the exe installer (3.7) and
so is incompatible with the QSWAT+ installer distributed with the SWAT+ Editor (see below). It is, however, possible to use QSWAT+ with this QGIS installer, by selecting a QSWATPlus3_9 installer from
https://bitbucket.org/ChrisWGeorge/qswatplus3/downloads/.
You will need to install the SWAT+ Editor as described below and then add the QSWATPlus3_9 plugin. You would then select QSWATPlus3_9 from the plugins menu of QGIS instead of 3_64. If you select a plugin incompatible with your version of QGIS the plugin will
appear in the plugins menu but the QSWAT+ button will not appear. You will find a message including the phrase "Failed to load Cython module: wrong architecture?" in the QSWAT+ tab of the QGIS log messages.
Other solution is install QGIS-OSGeo4W-3.16.11-1-Setup-x86_64.exe and swatplus-windows-installer-2.0.4.zip. Both in Python 3.7
I hope this help.