Soccer Ball Stealing System?

Your code:

script.Parent.Touched:connect(function(player)
    if player:FindFirstChild("Humanoid") then
        for _,v in pairs(game.Players:GetPlayers()) do
        v.Backpack:ClearAllChildren()
        wait (0.1)
        ball:clone().Parent = player.Backpack
        end
    end
end)

When the ball is touched it doesn't tag a model, it tags the part that touched it. So at line two where you put

player:FindFirstChild("Humanoid")

The machine is searching player2's body part that touched the ball for a humanoid, but the humanoid is inside the player's character model not their body part.

 for _,v in pairs(game.Players:GetPlayers()) do
        v.Backpack:ClearAllChildren()
        wait (0.1)
        ball:clone().Parent = player.Backpack
end

You don't need to remove everything from everyones backpacks or clone the ball, this could cause some problems. All you need to do is parent the ball to player2's character

New code:

--Make sure the script is inside the Handle of the ball
debounce = false
script.Parent.Touched:Connect(function(part)
    local player = game.Players:FindFirstChild(part.Parent.Name)
    if player and debounce == false then
        debounce = true
        ball.Parent = player.Character
        wait(1) -- time before the ball can be taken again
        debounce = false
    end
end)

I also added a debounce so the ball can't be taken again immediately after someone else takes it. They have to wait at least a second, this is useful or the ball might continuously loop between the two players.

Based on your previous script

Improvements

Use :GetService() to retrieve the Players service

You don't need to clear the children of the backpack as the tool may be in the player's Character (as it is when equipped)

Use :GetPlayerFromCharacter() to differentiate between players and NPCs

Issues

You're cloning the ball and parenting the clone, so the original ball is not having its parent shifted / being destroyed

Revised Code

local debounce = false

handle.Touched:Connect(function(hit)
    if hit and hit.Parent and game:GetService("Players"):GetPlayerFromCharacter(hit.Parent) then
        if hit.Parent ~= handle.Parent.Parent and debounce == false then
            debounce = true
            handle.Parent.Parent = hit.Parent
            wait(1)
            debounce = false
        end
    end
end)

debounce for cooldown, check the hit.parent is not already the tool's parent

Revised Server Script

local repst = game:GetService("ReplicatedStorage")
local remote = Instance.new("RemoteEvent")
remote.Name = "FootballEvent"
remote.Parent = repst

local debounce = false

remote.OnServerEvent:Connect(function(player, event, tool)
    if event == "Attach" then
        local char = player.Character
        local root = char:WaitForChild("HumanoidRootPart")

        local handle = repst:WaitForChild("Handle"):Clone()

        local weld = Instance.new("Weld")
        weld.Part0 = root
        weld.Part1 = handle
        weld.C0 = CFrame.new(0, -2.5, -1.5)
        weld.Parent = handle

        handle.Parent = tool

        handle.Touched:Connect(function(hit)
            if hit and hit.Parent and game:GetService("Players"):GetPlayerFromCharacter(hit.Parent) then
                if hit.Parent ~= tool.Parent and debounce == false then
                    debounce = true
                    tool.Parent = hit.Parent
                    wait(1)
                    debounce = false
                end
            end
        end)
    elseif event == "Remove" then
        local handle = tool:FindFirstChild("Handle")
        if handle then
            handle:Destroy()
        end
    end
end)