K.Nari

I also have the same problem.
Because the path has not been set, this problem has occurred probably

debug code

public function wpvp_check_extension($ext){
        $ffmpeg_path = get_option('wpvp_ffmpeg_path','');
        if($this->wpvp_check_function('exec')){
                if($ext=='ffmpeg')
                        $ext = $ffmpeg_path.$ext;
                exec("which ".$ext . " 2>&1",$output);
        foreach($output as $vv){
                 echo($vv . '<br>');

                }
                return $output;
        } else {
                return 'exec is disabled on the server';
        }
}

result
which: no /usr/bin/ffmpeg in ((null))

I think the problem is solved if you set the PATH.