Verify the files located in your SWATPlus installation folder, SWATPlus/SWATPlusEditor/resources/app.asar.unpacked. The contents should be as described below:
api_dist
swatplus_api.exe
swatplus_rest_api.exe
swat_exe
There should be several executable files matching the current model revision number, e.g.,rev60.5.2_64rel.exe
appsettings.json
swatplus_vardefs.sqlite
If any or all of the files above are missing, your installation did not complete successfully. You might try disabling your antivirus and installing again. Otherwise, download the zip file linked here and replace the resources folder entirely. Extract the zip to SWATPlus/SWATPlusEditor.