Hardcoding FFMPEG subtitles on MP4 / MKV
Solution 1:
sub2video: non-bitmap subtitle
That's your error. You have text-based subtitles, so you need to burn them with the subtitles filter.
ffmpeg -i input.mp4 -filter:v subtitles=subtitles.srt output.mp4
The overlay filter only works when your subtitles are images, which is the case for dvdsub, for example.
Furthermore, the problem seems to be that your subtitle file is not formatted correctly. Your time stamp definition contains an en-dash (–) instead of a normal minus-hyphen (-):
–->
Replace it with a correct minus-hyphen:
-->