You need to pull that game.ReplicatedStorage.ChangeData.OnServerEvent:Connect() out of game.Players.PlayerAdded:Connect(). You’re creating a new connection to ChangeData every time a player joins, which will create problems, so place that connection outside of the other so they are connected once each.
You can use numbers AFAIK but following certain syntax, like dictionaries. Sounds like this is your problem:
local table = {89 = something}
89 can’t be an index, but this can:
local table = {["89"] = something}
@mlnitoon2 You need to change the code like this game.ReplicatedStorage.ChangeData:FireServer(save.nickname)
i Hope this work ! 
but i need it to have the player
oh wait i have idea
i can make a variable of plr make it to nil
then set it to player
stilll errors the same thing… 08:47:52.279 104: Cannot store Instance in data store. Data stores can only accept valid UTF-8 characters. - Server - leaderstats:28
You can’t save an instance into datastore, only tables. What’re you trying to save the player for?
Store the player’s name/user ID instead, you can’t store the player instance itself.
the name they wanted the account name to be…
How are you creating the table? You can save the name as a string variable in it, but somewhere you’re trying to reference an Instance.
dang i worded this entire question a few months ago, what i ment was how do i make a login/sign up system