I was able to generate a gif without subtitles. Though it fails when you try to make a gif with subtitles.

Considering that you

• added FFmpeg to Windows path and;
• dir – the output directory exists

Change lines 138 and 139 to
local file_name = mp.get_property_native("filename/no-ext")
local file_path = output_directory .. "/" .. file_name

Hope this helps :))

After change line 138 and 139, error is gone but instead gif doesn't generate like #9. I able to generate gif when run mpv in Sandboxie though. Can't figure out why either.