Are you able to open the Standalone version? remember that you need iLok License Manager to activate/run the Plug-In.
We recently updated Archetype: Plini. You could try uninstalling the current version to install the new one. Download it from here: https://neuraldsp.com/downloads/
If it still doesn’t show up, go to C:\ Users \ Your profile \ AppData \ Roaming \ REAPER, and open the reaper-vstplugins64.ini file. Look for the line that begins with “ArchetypePlini.dll” and delete it, save changes and close the file. After that, open Reaper and re-scan your Plug-Ins again.