From your screenshot, it appears that you are running VirtualBox on OS X - is that correct?
If so, by default - a user account can not create IP endpoints that listen on ports below 1024. You are left with two choices if you absolutely need to have something listening on a low port number:
Disclaimer: I don't have a Mac in front of me, let alone a recent copy of OS X - so while the high-level ideas still apply, the actual details of configuring the firewall forwarding rules may be slightly different depending on which version you are currently running.
There may be better ways still to handle this. For example, if you configure a second virtual adapter for your guest and configure it as "Host-Only" you can then access any of the ports on the guest from your host without having to forward any of the ports. This may not make sense if you need to share the guest across your network - in which case perhaps the "Bridged" virtual adapter would make more sense. I would suggest reading up on the different networking modes in Chapter 6 of the VirtualBox Manual: https://www.virtualbox.org/manual/ch06.html