If you are running a current OS it is best to run the VST3 versions exclusively.
If you are having trouble loading any VST version, delete the VST2 or VST3 files from your computer and rescan the plug-in folders from your DAW preferences. You will find common plugin folders in Uninstalling SSL plugins.
VST3 supersedes VST2, and current SSL plugins do not include legacy VST, only VST2 and VST3. If you need legacy VST, you must install Duende Native V5 or SSL Native V6.
Studio One can run AU, VST2, and VST3. As stated it is best to run VST3 exclusively.
Changing or renaming folders in the Studio One plugin menu and rescanning can find missing plugins.
Common VST folders
C:\Program Files\Common Files\VST3
C:\Program Files (x86)\Common Files\VST3
C:\Program Files\Common Files\VST2
C:\Program Files\Common Files\Steinberg\VST2
C:\Program Files (x86)\Steinberg\VstPlugins