How to load ttml subtitles?

[hhh333]

I downloaded mp4 file with ttml type subtitle file. 

I'm assuming it is an external file.

My bad.

Code:

ffmpeg -i file.ttml -c srt file.en.srt

Should work just the same.

I get

Code:

Invalid data found when processing input

when trying the command. I also tried the same command with different ttml file and I got the same issue. I run it on RPi and am on the latest version. Also I couldn't find this in ffmpeg Documentation. I tried to list subs with ytdlp but it offers only the one file. I also tried python script with webvtt and pycaption but none worked so far.