How Do I make a script to change everyone's walkspeed?

I'll Explain the script, So you understand what to do next time; Don't use game.Players.LocalPlayer Because that's a technique which only works well in a LocalScript. So Instead we use : game:GetService'Players'.LocalPlayer And now that that's done, I'll Explain it, We don't Define Players from the Workspace unless you want to Define specific players, but since you want everyone who touches this part to go faster we'll use this method. Ok Here's our script:

local BasePlate = script.Parent -- Our Trigger
function onTouch(Brick) -- Declared a onTouch function
    local Player = game:GetService'Players'.LocalPlayer -- Our Player
    if (Player ~= nil) then -- Check to see if it exists
        Player.Character.Humanoid.WalkSpeed = 32 -- Changing the walkspeed
    end -- ending our if statement 
end -- ending our funtion
BasePlate.Touched:connect(onTouch) -- connecting out function to our Trigger

there is our another way which is faster, I don't need to Explain it because it's exactly the same but just less lines of Code.

local BasePlate = script.Parent -- Our Trigger
function onTouch(Brick) -- Declared a onTouch function
    local Player = 
    Brick.Parent:findFirstChild("Humanoid") -- Our Player
    if (Player ~= nil) then -- Check to see if it exists
        Player.WalkSpeed = 32 -- Changing the walkspeed
    end -- ending our if statement 
end -- ending our funtion
BasePlate.Touched:connect(onTouch) -- connecting out function to our Trigger

Hope I helped, If it worked Mark as answer :D Also if you need any help or if it doesn't work just comment below and I'll be happy to help.

LocalPlayer is a property of the Players service... therefore ...not located in the workspace, and can only be used by LocalScripts.

You'd probably be better off using a server script though.

local speed = 10

game.Players.PlayerAdded:connect(function( user)
    --newly added player == `user`

    user.CharacterAdded:connect(function( char )
        --user's character(workspace>user) loaded
        local humanoid = char.Humanoid

        if humanoid then
            humanoid.WalkSpeed = speed
        end;
    end);
end);