Error: The type of file 'subtitle_name.srt' could not be recognized

[decoyer]

Hi, to day I got some proble with muxing srt subtitle into mp4 files
Here is the command that I ran
mkvmerge

 mkvmerge --output "toMux/$destMux" "toMux/$fileName" --language "0:$subLang" --track-name "0:$subLang" "toMux/$subName"

I'm pretty sure that I got all input right, mp4 file and srt file definitely exist, but for some reason I can't muxed them. WHen I tried with ffmpeg with -i flag, it also say the similar things "error processing input"
Can someone please suggest what can I do? My code work once with the other series just a few minutes ago, it can mux subtitles to video just fine. But I stumble on this one, my be even this series