Cause Analysis:
The colon : is present in the full path of the font file.
If there is a path:
C:/ffmpeg/fonts/SourceHanSansCN-Regular.ttf
There is a colon after the C character, it will be recognized by ffmpeg as a separator, you need to use \\ to escape the colon.
Just change the above path to the following:
C\\\:/ffmpeg/fonts/SourceHanSansCN-Regular.ttf
The complete code is as follows:
-vf drawtext="fontfile=C\\\\:/ffmpeg/fonts/SourceHanSansCN-Regular.ttf:text='test'"