Garry's Mod Server Help(Port Forward and Srcds Help)

Hi everyone, my name is Greg and I have a problem. From the title you can tell it is a Gmod Server problem. I have played Gmod for a long time and wanted to make my own server for friends and other people. I downloaded Srcds for the website and installed it. It works. I then had to Forward my ports. I did these:

27015-27015-UDP/TCP(my Ip(192.168..))

I try to start Srcds,now it says “Can’t Allocate any IP Port”

Help please.