Your CPU and motherboard are not compatible.

To further explain, Z370 is LGA 1151 v2. It was made to have a different pin layout than previous LGA 1151 CPUs and sockets. So the only CPUs compatible with Z370 chipset are 8th-gen Intel. G4400 is 6th-gen. You'd need an i3-8100 to be compatible. Which may be worth it considering motherboards that support 6-GPUs for 6th and 7th-gen Intel are hard to come by these days.