when i press a part, the gui changes btw heres the script
local function click(player)
game.Workspace.Countdown.BrickColor = BrickColor.new("New Yeller")
player.Character.Head.Anchored = true
wait(10)
game.Workspace.Countdown.BrickColor = BrickColor.new("Lime green")
player.Character.Head.Anchored = false
local ReplicatedStorage = game:GetService("ReplicatedStorage")
local remoteEvent = ReplicatedStorage:WaitForChild("RemoteEventGuiColor")
remoteEvent:FireServer()player.PlayerGui.TasksBar.t2.BackgroundColor3 = Color3.new(0.0980392, 1, 0)
remoteEvent.OnServerEvent:Connect(player)
end
workspace.Task1.ClickDetector.mouseClick:connect(click)

2 comments
As the error suggests, fire server can only be called from the client( change fireserver to fireclient)
Also please make an attempt when trying to solve this issue. Otherwise, your questions will be closed.