Screens Connect displays a "port mapping failed" message under the following circumstances:
The router or modem providing the computer with Internet access (the gateway) doesn't support UPnP or NAT-PMP. See Manually Configuring a Port in Screens Connect.
The gateway's UPnP or NAT-PMP service is unreliable. Check the manufacturer's Web site for a firmware upgrade, or see Manually Configuring a Port in Screens Connect.
If you're running Screens Connect on a computer which is accessing the Internet through a wireless "hot-spot" (e.g. iPhone with Personal Hotspot enabled), Screens Connect will be unable to automatically map a port, and will display this error.