Audacity is a 32 bit application and I believe it only supports 32 bit plug-ins (even on a 64 bit system).
There is a simple to use Limiter that is a “Nyquist plug-in”. It is available here: https://forum.audacityteam.org/t/peak-limiter/20300/1
Instructions for installing: http://wiki.audacityteam.org/wiki/Download_Nyquist_Plug-ins#Installing_Plug-ins