The boots you are wearing, or the cosmetic that’s applied?
My translation had it a bit different but from what it says, the type of gear doesn’t match what the cosmetic wants to apply. But I have no idea what exact piece of gear you might be having an issue with…
You may need to test by un-equip each piece (by taking it off your character and putting it in your character inventory) one at a time and see if you can apply the cosmetic to the rest of the items.
One or more item(s) might not be the right type of item to apply the cosmetic to.