Attempt to compare number with nil?

tonumber() returns nil if the argument can't be converted into a number, for example, if it's empty.

Try this:

box = script.Parent

while wait() do
    number = tonumber(box.Text)
    if number and number > 90 then
        box.Text = 90
    elseif number and number < 0 then
        box.Text = 0
    end
end

If you use tonumber on a string that can't be converted to a number, it'll return nil.

print(tonumber("Hello, world!")) -- nil

Therefore, you can simply check if it returns nil.

-- I converted all your variables to local variables. Always use local variables.

local box = script.Parent

while wait() do
    local number = tonumber(box.Text)
    if number then -- this checks if number isn't nil
        if number > 90 then
            box.Text = "90" -- set text to a string
        elseif number < 0 then
            box.Text = "0" -- set text to a string
        end
    end
end

There are ways you can make this code better by using events, particularly the Changed event, or GetPropertyChangedSignal thingy, but for now it's probably not that big of a deal.