Ceiling fan requires fewer wires than available in box
In many homes, boxes wired for ceiling fans will have one wire that is always hot, and another that is connected to a switch.  In my experience, the black wire is switched and the red is always hot, but that may not be standard and you would need to test your wires.

If you want the fan to be "always on" and only controlled with the remote, then you want to use the always hot wire and cap the other.  If you'd rather have the fan turned on and off with the switch, and use the remote when the switch is on, you can use the switched wire and cap the other.

The wire color will have to be tested by you, and the choice of which to use is yours.