Trying to change walkspeed on CharacterAdded...?

Try this:

Player = script.Parent.Parent.Parent.Parent.Parent
Cost = 1 

script.Parent.MouseButton1Click:connect(function()
if Player:findFirstChild("skills") then
if Player.skills.Talents.Value >= Cost then Player.skills.Talents.Value = Player.skills.Talents.Value -Cost
if(player.Character:FindFirstChild("walkAddition") == nil) then
local player = script.Parent.Parent.Parent.Parent.Parent
local human = player.Character.Humanoid

local walkAddition = Instance.new("NumberValue",player)
walkAddition.Name = "walkAddition"
walkAddition.Value = 2.5
end
script.Parent.Parent.DasherBought.Visible = true
end
end
end
end)

game.Players.PlayerAdded:connect(function(plr)
plr.CharacterAdded:connect(function(character)
local walk = plr:FindFirstChild("walkAddition")
if(walk ~= nil) then
character.Humanoid.WalkSpeed = character.Humanoid.WalkSpeed + walk.Value
walk:Destroy()
end
end)
end)

Basically what happens, is if the button is pressed it adds an object to the player, and when the character respawns it detects if the object is there, and if it is it'll change the walk speed and delete it!

If you need any more help just ask! :D

This should help.

`` ****NOTE:**** I typed this in the scripting helpers website, so there may be some capitalization errors. I also did some code-writing cleanups.

`` ****IMPORTANT!**** This script looks like it needs to be in a client-side (Local) script. Make sure it is there.

--[[
**HOW TO make good code:**
- Indent!
- If you happen to be sharing on Scripting Helpers, comment (using "--" for short (rest of line) comment, and "--[[ to open a long comment, which goes on until "]]--" is used.)!
- Use some print functions to tell you what happened in the code for easy troubleshooting!
- You made a player variable, so use it!
- Please, use "local" before your variables.
]]--

---------------------------------
---------[ Variables ]--------
---------------------------------

local Player = script.Parent.Parent.Parent.Parent.Parent
local Cost = 1 
local hasGivenBoost = false -- used in the "once only" section.

----------------------------
---------[ Code ]--------
----------------------------

script.Parent.MouseButton1Click:connect(function()
        if Player:FindFirstChild("skills") then
            if Player.skills.Talents.Value >= Cost then Player.skills.Talents.Value = Player.skills.Talents.Value -Cost
            print(Player.Name.."'s talent is the greater than the cost.")
            Player.CharacterAdded:connect(function(c) -- detects when the player's character was added (every time)
                --[[
                -- Remove then long comment things ("--[[" and "]]--") if you only want it to do it once.
                If hasGivenBoost = false then
                    hasGivenBoost = true
                    print(Player.Name.." just got the speed boost.)
                else
                    print(Player.Name.." tried to get it after they already got the boost")
                    return
                end
                ]]--
                print(Player.name.."'s character was added")
            c.Humanoid.WalkSpeed = c.Humanoid.WalkSpeed + 2.5 -- Add the character's speed boost
                script.Parent.Parent.DasherBought.Visible = true -- turns the thingy that says that it was bought to be able to be seen.
            end)
        end
    end
end)