How to give certain people walkspeed?

WalkSpeed is a property of the Humanoid.

If you want to set a specific persons walkspeed to something, use an if-statement to check if the player's userId is equal to the specific player your looking for.

This is an example that'll give you a rough overview of how setting walkspeed works in general:

local Players = game:GetService("Players")
local SET_WALKSPEED = 32

for _, Player in pairs(Players:GetPlayers()) do
    local Character = Player.Character or Player.CharacterAdded:wait()
    if Character then
        local Humanoid = Character:FindFirstChild("Humanoid")
        if Humanoid then
            Humanoid.WalkSpeed = SET_WALKSPEED
        end
    end
end

Put this in a ServerScript, then you can either put the ServerScript in ServerScriptService or the Workspace.

local SpecialPeople = {} -- Paste your wanted player's userid inside the empty table.
local CustomWalkspeed = 20 -- Your preferred walkspeed. 

game.Players.PlayerAdded:Connect(function(Player)
    Player.CharacterAdded:Connect(function(Character)
        for i = 1, #SpecialPeople do
            if Player.UserId == SpecialPeople[i] then
                warn(Player.Name.." has joined the game.")
                local Humanoid = Character:WaitForChild("Humanoid")
                Humanoid.WalkSpeed = CustomWalkspeed
            end
        end
    end)
end)

local certainpeople = {"PlayerName","Person"} -- put persons' names here
local certainid = {1234567, 2345678} -- put persons' user ids here
local takename = true -- put true if you want the player name to be identified.
-- put false if you want the player id to be identified
local speed = 25 -- change the value to the speed you want them to have

game.Players.PlayerAdded:Connect(function(player)
    if takename == true then
        for i, x in pairs(certainpeople) do
            if string.lower(player.Name) == string.lower(x) then
                player.Character.Humanoid.WalkSpeed = speed
            end
        end
    elseif takename == false then
        for i, x in pairs(certainid) do
            if player.UserId == x then
                player.Character.Humanoid.WalkSpeed = speed
            end
        end
    end
end)

Tell me in the comments if it doesn't work.