make a script and put it in ServerScriptService Parent the katana under the script
local Players = game:GetService("Players")
local Katana = script:WaitForChild("Katana")
Players.PlayerAdded:Connect(function(Player)
Player.CharacterAdded:Connect(function(Character)
local RootPart = Character:WaitForChild("HumanoidRootPart")
if RootPart then
local Clone = Katana:Clone()
local Weld = Clone.KatanaAttachPart.TorsoWeld
Weld.Part0 = RootPart
Weld.Part1 = Weld.Parent
Weld.C0 = CFrame.new(0, -RootPart.Size.Y/2, RootPart.Size.Z/2)
Weld.Part1.Anchored = false
Clone.Parent = Character
end
end)
end)