ffmpeg option - I have a question about c:v.

If you encode a raspivid video with a codec in ffmpeg's -c:v, the error Protocol not found appears.
The command is

ffmpeg test-y-i-c:vlibx264 test01.mp4

I'm doing it with this.The test is a video obtained by raspivid.
If you encode mp4 without using c:v, you can make mp4 videos.

I don't know why c:v causes Protocol not found.
I would like someone to tell me