Why is this line erroring?

I have a line in my code that is supposed to save a table that is erroring. The error is, "Cannot store Array in Datastore" Here is my script,

game.Players.PlayerAdded:connect(function(plyr)
    while true do
        wait(60)
        local build=script.Parent

        local pur=build.purchased
        local DataStore = game:GetService("DataStoreService")
        local PlayerData = DataStore:GetDataStore("PlayerData")
        local PlayerD
        local pnumb=#pur:GetChildren()
        local purs=pur:GetChildren()
        local cframes={}
        local number=0
        for i=1, pnumb do
            number=number+1
            local item=purs[number]
            local cframe=item.PrimaryPart.CFrame
            table.insert(cframes,number,cframe)
            print(item.Name)
            print(cframes[number])
        end
        PlayerData:SetAsync(("OBJECTS".. plyr.UserId),purs)
    end
end)

I have no idea why this is erroring as i thought that tables could be saved with datastore. An explanation could be good.