It may happen that Port Mapper is not able to create a rule you asked.
There are some steps you might try:
Is the port already in use?
If the port was already forwarded by another machine on your network, mapping may fail.
Is UPnP or NAT-PMP enabled on your router?
This is required for Port Mapper to work.
Unfortunately, this varies by router model so we are not able to assist you in this.