KiCads ERC checks which pin types are connected to which net. Every pin type has requirements for it to pass the check. Power inputs need to have a power output connected on the same net.
I guess you supply your system with a generic connector which means you somehow need to tell kicad where the power comes from. For this there is a special symbol in the power lib called the pwr_flag. It is a symbol with a single power output pin that can be used to fulfill ERC. More details see ErrType(3): Pin connected to some others pins but no pin to drive it