What fixed this for me was to change "fontsize" argument name to be "font_size" and add the keyword argument to the subtitles call:

make_clip = lambda txt: TextClip(font='/usr/share/fonts/liberation-sans/LiberationSans-Regular.ttf', text=txt, font_size=fontsize, color=color)

  
# Load the subtitles
subtitles = SubtitlesClip(srt_path, make_textclip=make_clip)