Audacity 2.3.1 (64-bit) requires the 64-bit versions of LAME and FFmpeg. If you believe that you have installed Audacity 2.3.1 and the 64-bit versions of LAME and FFmpeg correctly but have messed up Audacity’s settings somehow, then you could try resetting Audacity. To do that, you need to delete the “audacity.cfg” file. See this post for detailed instructions: https://forum.audacityteam.org/t/resetting-audacity/50613/1