How to make it so that the numbers don't round off?

There are different ways of doing this, but I'll explain the most simple and straightforward way.

Count down from 900 by 1, then divide by 10.

script.Parent.MouseClick:Connect(function()
    for i = 900,0,-1 do
        for _,p in pairs(game.Players:GetPlayers()) do
            p.PlayerGui.ScreenGui.Timer.Text = i/10
        end
        if i == 0 then
            for _,u in pairs(game.Players:GetPlayers()) do -- "u" is a metaphor variable that stands for "User"
                u.PlayerGui.ScreenGui.Timer.Text = "Failure!"
            end
            game.Workspace:FindFirstChild("Clear", true).Disabled = true
        end
    end
end)

Now I'll do you another favour and save you some future trouble. The Server cannot access PlayerGui. So it's best you do the counting client-side.

-- Local script inside the TextLabel
ClickDetector.MouseClick:Connect(function()
	for i = 900,0,-1 do
		script.Parent.Text = i/10
		wait(1)
	end
	script.Parent.Text = "Failiure!"
end)

Be sure to Upvote/Accept if I helped!