Hi and welcome to Reaper forums.
(This is assuming you're on Windows..)
Are you running Reaper 64 bit?
If so, seeing this is a 32 bit only plugin, you need to have the bridging feature enabled for it to work, this can be selected when installing Reaper.
Unless other 32 bit plugins you are using are running fine,I'd suggest reinstalling Reaper and make sure above option is ticked.