I have this too. I think it’s related to crafting a weapon with the assistance of a thrall. I crafted a hardened steel shield without a thrall and I was able to mod it but the ancient mace I built with a T4 damage thrall at an improved armoursmith’s bench refuses to let me mod it with either a damage kit or spiked kit…

EDIT: In my case it seems a mod is causing it: “No Repair Bulls***” as weapons crafted without it seem to let me mod them fine.