Rescanning plug-ins (macOS & Windows)
Make sure the Plug-in is installed on the correct folder.
After that, open Reaper, go to Options > Preferences > VST, Verify that Reaper is reading the VST folder, and click Clear cache/re-scan.
If the Plug-In still doesn't show up:
Close Reaper, uninstall the Plug-In, and reinstall it with the exception of adding a subfolder containing it (example: C:\Program Files\VSTPlugins\"Custom Folder Name").
Re-scan your Plug-Ins and check if the issue persists.
"I tried that and I cannot make it work yet..."
In Windows, go to C:\Users\Your Profile\AppData\Roaming\REAPER
Then, find the "reaper-vstplugins64.ini" file.
Open it and look for the line corresponding to the Plug-In you want to install (example: Archetype_Plini.dll=XXXXXXXXXXXXXXXX), it should appear as an incomplete line if you compare it to the others.
Delete it (including the space left by it), open Reaper, and re-scan your Plug-Ins.