You need a Xeon for ECC.

No, your i7-9700K won’t do ECC, even though the board says it supports it. Intel locks ECC memory support to Xeon CPUs on consumer platforms. The memory controller in a Core i7 simply doesn’t have the circuitry. The board can talk to ECC DIMMs, but the CPU will ignore the extra bits—they’ll just run as non-ECC.

If you want ECC, you need a Xeon E-2200 series, like the E-2288G. That’s the only way to get actual error correction on that board.

Check the CPU’s official specs before you buy anything. ECC support is always listed separately.