The most basic schematic doesn't not pass ERC check

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