Warzon (Call of Duty: Vanguard)

Good afternoon, I have a problem opening Sai melee weapon skins in Call of Duty Warzone. It is necessary with the help of this weapon to make finishing blows. I did this many times while holding the Sai weapon in my hands, but with the finishing blow itself, there is a blow that is selected in the game menu, but not like with the help of Sai. How do I complete this quest? Please, help.