After I create a path and run /cam start [time] it always shows: "No target found", I've tried doing "/cam target none" but it just doesn't work! :(

Which version are you using? Would also be helpful if you could send me your log file.

I'm using 1.15.2 and after doing some testing, it only doesn't work when i set the interpolation do "circular" and the log file; I believe this is the correct file
latest.log

The circular interpolation requires a target. Just stick to the default interpolation.
https://www.youtube.com/watch?v=mcmqgHNU5Vg