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.