You can't use an 8th Generation processor on a 7th Generation motherboard. You will need to get a motherboard has a chipset that is compatible with the 8th or 9th Generation CPUs.
Since there are no officially supported 7th Generation Intel processors that means you need both a motherboard and processor that are at least 8th Gen, or higher. For AMD you will need at least a 3000 series or newer CPU and a motherboard that supports fTPU.
There are ways to get around the CPU requirement but they are not officially supported and are not recommended as there are various problems that accompany that hack.