That is not theonly software required to convert video, you may have problem with other software, any way, try running the convert command on the server and see if it can convert video into flv format.

Code:
/usr/local/bin/ffmpeg -i /var/www/vhosts/profilespark.com/httpdocs/video/5.avi -acodec mp3 -ar 22050 -ab 32 -f flv /var/www/vhosts/profilespark.com/httpdocs/flvideo/5.flv