You can't mix ECC and non-ECC RAM

No.

ECC and non-ECC RAM don’t play together, even if the physical sticks fit the same slot. Your motherboard and CPU must explicitly support ECC — consumer boards almost never do. And even on the rare workstation board that does, mixing ECC and non-ECC is not a supported configuration. Most systems either ignore the ECC stick entirely or refuse to boot.

Best case: the motherboard runs with just the non-ECC pair (24GB). Worst case: it fails to POST or becomes unstable. Either way, you’re not getting a clean 40GB.

Stick with matching non-ECC sticks. The extra 16GB of ECC is a paperweight here.