How do i fix this data item saver?

When you read and overwrite data your not typing the correct function names.

Reading data

To read data you need to use GetAsync() not GetSync()

Example:

local data = game:GetService("DataStoreService"):GetDataStore("Inventory")

game.Players.PlayerAdded:Connect(function(plr)
    local SavedItems = data:GetAsync(plr.UserId)
end

Saving data

This is the same problem as reading data. You named the functions incorrectly. Sync() is not a function of DataStore. Instead of using Sync() use SetAsync() which will overwrite data.

Example:

game.Players.PlayerRemoving:Connect(function(plr)
    local Table = {}

    for i, v in pairs(plr.Backpack:GetChildren()) do
        table.insert(Table,v.Name)
    end

    if Table ~= nil then
        data:SetAsync(plr.userId,Table)
    end
end)

Here is a re-written version of your script:

local DataStoreService = game:GetService("DataStoreService")
local DataStore = DataStoreService:GetDataStore("Inventory") -- Separate variable

local function Load(plr)
    local key = "plr-"..plr.UserId -- key variable (The way I do it)
    local savedData -- Will be set below

    local success, err = pcall(function()
        -- This makes sure that the script will keep going if it errors
        savedData = DataStore:GetAsync(key) -- GetAsync()
    end  

    -- if datastore failed to read data
    if not success then
        warn("Failed to read data"..tostring(err))
        return -- this will return nil
    end

    -- if datastore succeeded 
    if savedData then
        -- if the player has data then load the data

        for i, v in pairs(savedData) do
            if v then
                local item = game.ServerStorage.Inventory:FindFirstChild(v):Clone()

                if item then
                    item.Parent = plr:WaitForChild("Backpack")
                end
            end
        end
    else
        -- If the player does not have data to be loaded

        local items = {}

        local itemsSaving = plr:WaitForChild("Backpack"):GetChildren()

        for i, v in pairs(itemsSaving) do
            if v then
                table.insert(items, v.Name)
            end
        end

        local success, err = pcall(function() -- protected call
            DataStore:SetAsync(key, items) -- SetAsync()
        end

        -- if overwriting data failed
        if not success then
            warn("Failed to overwrite data"..tostring(err))
            return
        end
    end
end

local function SaveData(plr)
    local key = "plr-"..plr.UserId -- Again just how I do it
    local items = {}

    local itemsSaving = plr:WaitForChild("Backpack"):GetChildren()

    for i, v in pairs(itemsSaving) do
        if v then
            table.insert(items, v.Name)
        end
    end

    local success, err = pcall(function()
        DataStore:SetAsync(key, items) -- SetAsync()
    end

    -- failed
    if not success then
        warn("Failed to overwrite data"..tostring(err))
        return
    end
end

-- Calls
game.Players.PlayerAdded:Connect(Load)
game.Players.PlayerRemoving:Connect(SaveData)

Now you might not need to use the pcalls but they sure came in handy to me. NOTE: "THIS SCRIPT IS NOT TESTED. IT COULD THROW ERRORS AND MAY NOT WORK." (Used caps lock not to be mean but to make sure you see that)

I wrote the script above to fix any bugs with your script. It should work just by copying the script but you may have some tweaking to do. Hope this helps!