Some process is already useing port 81,
netstat -tapn | grep '192.xxx.xxx.78:81'
to see which process is using port 81
stop that process, then start httpd, then start the process that was using port 81 before.
Besides that, I don't want to demotivate you, but if you're new to linux .. don't you think setting up a HA httpd cluster is a tad over the top atm? Ofcourse everyone can copy paste everything from these excellent how-to's and everything works (that's what makes this site so nice) .. but don't forget that you try and understand WHAT you're doing, so when problems start arrising, you have some knowledge of what you have and what could cause the problem. The problem you're posting is basic linux / network / server knowledge, maybe start a bit easier?