I checked in CS for "KO_Black_f_boot_f." of COV and I could not find it in the list. I also opened the COV in a separate folder and I could not find it there either. It's possible to forget little details when building big mods. Perhaps these boots are one of them. If that is the case, then perhaps a boot patch is in order.
If it isn't listed, then there wouldn't really be any way for a "foot" part associated with it to cause an error warning. No offense, but let me ask you this... You opened the CS with "BB_Clothiers of Vvardenfell" ticked as the active plugin, Then in the object window, you went down the list until you found the place where "KO was alphabetically next, yes?
So, if it isn't there, then the boots are calling the object and it isn't there... so after carefully checking in "body Parts" press the "clothing" tab on the object window. Go down the list until you come to "KO_Black_fold_boots_long" press on that and open it. When the window opens, look on the right side of it and see what it says in the window under the "female clothing" window.
Under "biped object, it should say "left foot" and beneath that "right foot". On the far right there should be two windows that read the same, "KO_Black_f_boot_f". If it reads as anything else, let me know, and we;ll make you a patch.