my error is: Expected ')' (to close '(' at line 20), got <eof> how do i fix that?

Problem:

The error indicates that an end is missing from a function, and in this case, it'd be line 20, based on the error line.

Information:

On line 20, there's no end) function to end the argument list of :Connect. Theend) is a callback from the argument, since it ends that function with a closing parenthesis.

Solution:

All you have to do is add an end) on line 20. Since that's where the error line in the output is stating.

Fixed Script:

-- Sprint Key Pressed
userInputService.InputBegan:Connect(function(input)
    if input.KeyCode == Enum.KeyCode.LeftShift then
        replicatedStorage.RemoteEvents.Sprint:FireServer("Began")
    end
end)

-- Sprint Key Released
userInputService.InputEnded:Connect(function(input)
    if input.KeyCode == Enum.KeyCode.LeftShift then
        replicatedStorage.RemoteEvents.Sprint:FireServer("Ended")
    end
end)

-- Update Stamina GUI
replicatedStorage.RemoteEvents.StaminaUpdate.OnClientEvent:Connect(function(stamina, maxStamina)
    players.LocalPlayer.PlayerGui.Stamina.Bar.Size = UDim2.new((stamina / maxStamina) *  .971, 0,  .247, 0)
end)

Reference:

RBXScriptConnection - API Documentation

Any more edits are gonna be improvements on the wording of the answer.