Where do you have the Kontakt VST. You need to make sure the VST directory is added in Reaper. I cannot get at my Reaper for the moment I have Thai visitors sleeping in my music room. Go to your preferences in Reaper, then under Plugins - VST add the path to the area where you have Kontakt.
In REAPER, press [Ctrl] + P (Windows) / [Cmd] + [,] (Mac) to access Preferences.
Go to Plug-ins > VST.
Under “VST Plugin Path,” make sure that the path is listed (if not, add it):
For example Windows: C:\Program Files\Common Files\VST
Click on Clear Cache/Re-Scan.
See
https://m.youtube.com/watch?v=CByHPXk3FlwHope you get it
Tony