How to make 4 digit code have no repeating numbers?

One way to do this is to make an array of digits from 0 to 9

Digits = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9}

We take a random number from Digits, and then remove said number since so that it cannot appear again

RandoNum = math.random(1, #Digits)
Chosen = Digits[RandoNum]
table.remove(Digits, RandoNum)

Then we can just take said number and add it to startCode.Value

If you wanted your code to be in a loop, it would look something like this:

Digits = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9}
for i = 1, 4 do
    RandoNum = math.random(1, #Digits)
    Chosen = Digits[RandoNum]
    table.remove(Digits, RandoNum)
    startCode.Value = startCode.Value..tostring(Chosen)
end

Start by making a array/table for every number in the number combo.

local code = {"N","N","N","N"}

Now, let's make a for loop for every part of the table. If there's no value set, ("N") we'll change it to a random number. Next let's make sure no other number in the array/table is that number.

local function check()
 for i, v in pairs(code) do
    if v == "N" then
            return true
        end
end
end
while check() do
    for i, v in pairs(code) do
        if v == "N" then
            local suggestednum = math.random(0,9)
            print(suggestednum)
            local isnotothernumber = true
            for i, v in pairs(code) do
                if v == suggestednum then
                    isnotothernumber = false
                end
            end
            if isnotothernumber then
                code[i] = suggestednum
            end
        end
    end
-- if you want it to be a string
 combostr = ""
 for i, v in pairs(code) do
    combostr = combostr..v
 end
end 

So there you are! A randomly generated number sequence that is 4 digits long and 2 digits will never be the same.