Variable Concatenation is nil?

In your code, you used "if PlayerReputation then". This makes it so if it ISN'T nil it will set to 0. So the value is never set to 0, hence it always being nil. A few more mistakes I also fixed. (Thanks @Spongocardo for catching those).

local DataStoreService = game:GetService("DataStoreService")
local GameData = DataStoreService:GetDataStore("Reputation")

game.Players.PlayerAdded:connect(function(Player)
    print(Player.Name .. " has joined the game")
    print("Geting Reputation for " .. Player.Name)
    local PlayerReputation = GameData:GetAsync(Player.Name .. "Reputation")
    if not PlayerReputation then -- Added not.
        print(Player.Name .. " has no Reputation... it was set as 0")
        GameData:SetAsync(Player.Name .. "Reputation", 0) -- Fixed spelling mistake
        PlayerReputation = 0 -- Removed local
    end
    print(Player.Name .. "'s Reputation is: " .. PlayerReputation)
    local Reputation = Instance.new("IntValue")
    Reputation.Name = Player.Name
    Reputation.Value = PlayerReputation
    Reputation.Parent = game.ServerStorage.PlayerRep
    print("Reputation has been loaded for " .. Player.Name)
end)

game.Players.PlayerRemoving:connect(function(Player)
    local SavedReputation = GameData:GetAsync(Player.Name .. "Reputation")
    local Reputation = game.ServerStorage.PlayerRep[Player.Name].Value
    if SavedReputation ==  not Reputation then
        GameData:SetAsync(Player.Name .. "Reputation", Reputation) -- Fixed spelling mistake
    end
end)