Altium: error since net connects only to Output Port objects
A named net that does not have a DRC suppression (the X on the unused pins) is considered an error; the tool (indeed, every tool I have ever used) expects a net to connect to at least 2 pins.

When this is used by a higher level (and the pin is connected to something else) the error should disappear.

To fool it for now, you could put a pullup / pulldown resistor on the net to eliminate the error.

See this article on reporting single pin nets and their definitions.