The LUA interpreter utilized by Roblox is flawed when it comes to instancing.
The interpreter probably thinks you’re referring to stamina as if it is a property of UI.
Instead, I recommend writing your code like this:
local GUI = script.Parent
local main = GUI.Main
local ui = main["UI"]
local border = ui["border"]
local health = ui["health"]
local stamina = ui["stamina"]
stamina.Text = tostring(math.floor(humanoid.status.block.stamina.Value * 100) / 100) .. "/100"