What will happen if I mix DDR4 RAM and DDR5 RAM in my PC?

Rig won't POST with all 4 RAM slots occupied Local tech guy thinks it's because I don't have a second ATX 8 Pin plugged in and wants me to buy

DDR4 is not backward-compatible with DDR3 so a DDR4 DIMM will not fit on a DDR3 DIMM slot Not only is the key notch of each DDR generation

ddr5 in ddr4 slot DDR5 series · DDR4 series · Industrial Motherboards · ATX · Micro ATX · Mini-ITX · 1 PCIe x16 slot; 4 SATA3, 2 x ; Graphics Output

ddr5 slot No The sticks are physically different DDR4 wont fit in a DDR5 slot and vice versa