How do I save a table of a player's backpack in a datastore?

I know there was already a question somewhat like this, but I didn't fully understand the answer. The error that I get from this script is that you can't store an array in datastore. So how would I store a table of the player's backpack in a datastore?

local DSS = game:GetService("DataStoreService")

local datastore = DSS:GetDataStore("GeneralSaveData", "Players")

function generateDataKey(player)
    local ret = "uid_" .. player.userId
    return ret
end

function generateDataTable(player)
    local dataTable = {}
    for _, tool in ipairs(player.Backpack:GetChildren()) do
        table.insert(dataTable, tool.Name)
    end
    return dataTable
end


function saveDataForPlayer(player)
    local key = generateDataKey(player)
    local data = generateDataTable(player)
	datastore:SetAsync(key, data)
end

function inputDataToPlayer(player, data)
if data then
for _,T in pairs(data) do
    local wep = game.ServerStorage.Weapons:FindFirstChild(T)
    if wep then
        wep:Clone().Parent = player.StarterGear
        wep:Clone().Parent = player.Backpack
    end
end

end
end
function loadDataForPlayer(player)
    local key = generateDataKey(player)
    local data = datastore:GetAsync(key)
    inputDataToPlayer(player, data)
end

game.Players.PlayerAdded:Connect(function(player)
    loadDataForPlayer(player)
    end)

game.Players.PlayerRemoving:Connect(saveDataForPlayer) 

There's probably other errors in the code that I didn't notice, so if you see any feel free to correct them.

Datastores can handle lists/tables (so long as you don't mix string-keys and integers in the same table), but they cannot handle Instances. I see that your script is already prepared to handle the datastore saving the names of tools (for when you load them, I mean), you just need to convert the player's Backpack's children to a list of names:

function generateDataTable(player)
	local dataTable = {}
	for _, tool in ipairs(player.Backpack:GetChildren()) do
		--might need an if statement here so that you only add to dataTable if 'tool' is something you want them to be able to save (ex if it's a gear item they brought with them, you wouldn't want only want that saved)
		table.insert(dataTable, tool.Name)
	end
	return dataTable
end

Other notes:

On line 30 you use a , to try and save a weapon in two places, but that's not how lua/Roblox work - you have to break it down more:

wep.Parent = player.StarterGear
wep:Clone().Parent = player.Backpack

--Also, to prevent having to find the weapon 2x, your loop could be written:

for _,T in pairs(data) do
	wep = game.ServerStorage.Weapons:FindFirstChild(T)
	if wep then
		wep:Clone().Parent = player.StarterGear
		wep:Clone().Parent = player.Backpack
	end
end

You may also want to improve your script to use pcall around GetAsync/SetAsync, in case they fail (though this does complicate things).