Localhost traffic doesn't appear in Charles
Some systems are hard coded to not use proxies for localhost traffic, so when you connect to http://localhost/ it doesn't show up in Charles.
The workaround is to connect to http://localhost.charlesproxy.com/ instead. This points to the IP address 127.0.0.1, so it should work identically to localhost, but with the advantage that it will go through Charles. This will work whether or not Charles is running or you're using Charles. If you use a different port, such as 8080, just add that as you usually would, e.g. localhost.charlesproxy.com:8080.
You can also put anything in front of that domain, e.g. myapp.localhost.charlesproxy.com, which will also always resolve to 127.0.0.1.
Alternatively you can try adding a '.' after localhost, or replace localhost with the name of your machine, or use your local link IP address (eg. 192.168.1.2).
If Charles is running and you're using Charles as your proxy, you can also use local.charles as an alternative for localhost. Note that this only works when you're using Charles as your proxy, so the above approaches are preferred, unless you specifically want requests to fail if not using Charles.