To change UI from the server you should use a RemoteEvent. Use it to send the data to the player, and let the player update the UI locally.
On the server:
On the client:
game.Players.LocalPlayer.PlayerGui.ScreenGui.Coins.Text = value
Another option is to let the player itself detect the coins changing, that way you wouldn’t need to worry about it on the server. Somehting like this (local script):
local player = game.Players.LocalPlayer
local coinValue = player:WaitForChild("leaderstats"):WaitForChild("Coins")
player.PlayerGui.ScreenGui.Coins.Text = coinValue.Value