Unable to convert Avisynth script in ffmpeg
From FFmpeg source code:


  On Windows, FFmpeg supports AviSynth interface version 6 or higher.
  This includes AviSynth 2.6 RC1 or higher, and AviSynth+ r1718 or
  higher, and excludes 2.5 and the 2.6 alphas.


From doom9:


  The decision to drop support for anything below 2.6 RC1 was due to
  catastrophic header incompatibilities between 2.5 and 2.6 RC1* and the
  maintenance burden (and potential licensing minefield that could mean
  any additional 2.5 support would violate GPL) that would be incurred
  by trying to continue to support 2.5 while using properly updated 2.6
  headers.




Get AviSynth+ r2420 from 
https://github.com/pinterf/AviSynthPlus/releases/tag/r2420-MT