error - ServerScriptService.print:2: attempt to index nil with 'Character' what i can do ?

You’re trying to reference a Local Instance from a Server, that of which is a global space; the Server doesn’t have a "LocalPlayer". This Object path can only be Index through a LocalScript as these programs operate on the Local Machine, which means they’re aware of which Client they are.

This isn’t much of a roadblock though, fortunately, the .OnServerEvent signal contains the Player Object that invoked the Event, we can simply just use that:

local ReplicatedStorage = game:GetService("ReplicatedStorage")
local RemoteEvent = ReplicatedStorage:WaitForChild("Print")

RemoteEvent.OnServerEvent:Connect(function(Player)
    local Character = Player.Character or Player.CharacterAdded:Wait()
    local Humanoid = Character:FindFirstChildOfClass("Humanoid")
    if (Humanoid) then
        Humanoid.WalkSpeed = 100
    end 
end)

If this works, don’t forget to accept this answer!

Try this code:

game:GetService('ReplicatedStorage').Print.OnServerEvent:Connect(function(plr)
hum = plr.Character:WaitForChild("Humanoid")
hum.WalkSpeed = 100
end)