The solution is fairly simple:
If you don’t have any power generating symbol in your schematic (any symbol with an power out pin) the DRC will complain that the power nets are not driven.
So for example when you use a connector for your power, the connector is a generic part with passive pins. The DRC can not know, that these pins in your schematics are the power outputs.
For these cases there is a special part in the library called PWR_FLAG. Just attach it anywhere on your +5V (and GND) net.
For an external power Input it might look like this: