Why can't I open a port on my PC!?

It's possible your ISP has blocked incoming traffic on the port in question to prevent file sharing. Even if you open your router, the request to the WAN address still has to go through your ISP's router and if they have it blocked you are out of luck. Check with your ISP to see if it's blocked. You can try a different port if the file sharing tool allows you to and the ISP allows incoming traffic.