chosenTable was out of scope (meaning the variable doesn't exist in the current context.)

Looking at the rest of your code, it seems like you're trying to alert the player that a table is taken.

If you were to put the code in an elseif, it would only run the code when the table number is either less than 1 or greater than 18, which isn't what we want to do.

We instead need to put the check to see if the table is empty inside the first if statement.

The logic would go like this:

player inputs a number

if the number is between 1-18, the table number is valid then check if the valid table is taken, alert the player that it's taken and let them try again.

if the table is not not taken, give that player that table

if the number is greater than 18 or less than one, the player typed in an invalid table. we tell them that it is invalid and let them try again.

Working Script:

LocalPlayer = game.Players.LocalPlayer
tableLots = script.Parent.Parent.Parent.TableLots

script.Parent.MouseButton1Click:Connect(function()
    local text = script.Parent.Parent.TableHandler.Text
    local number = tonumber(text)
    print("Number ", number)

    if number >= 1 and number <= 18 then
        local chosenTable = tableLots["Table"..tostring(number)]

        if(chosenTable.BackgroundColor3 == Color3.fromRGB(255,0,0))then
           script.Parent.Text = "Table already taken."
           wait(3)
           script.Parent.Text = "Please try again."
           wait(3)
           script.Parent.Text = "Submit"
        else
            chosenTable.BackgroundColor3 = Color3.fromRGB(255,0,0)
        end
    else
        script.Parent.Text = "There are only 18 tables.\n Please enter another table number."
    end
end)

There are a few things wrong.

-- these variables should be local because i'm assuming this isn't inside a function
local LocalPlayer = game.Players.LocalPlayer
local tableLots = script.Parent.Parent.Parent.TableLots

-- changed "connect" to "Connect", since "connect" is deprecated.
script.Parent.MouseButton1Click:Connect(function()
    local text = script.Parent.Parent.TableHandler.Text
    local number = tonumber(text)
    print("Number ", number)

    if number >= 1 and number <= 18 then
        local chosenTable = tableLots["Table"..tostring(number)]
        print("Table ", chosenTable)
        chosenTable.BackgroundColor3 = Color3.fromRGB(255,0,0)
-- elseif should be put together.
    elseif chosenTable.BackgroundColor3 == Color3.fromRGB(255,0,0) then
        script.Parent.Text = "Table already taken."
        wait(5)
        script.Parent.Text = "Please try again."
        wait(5)
        script.Parent.Text = "Submit"
        end
end)
-- You forgot the closed parenthesis

Haven't tested it, but fixed a lot of the formatting of this script. I'm assuming it's very old code. I also recommend using Activated instead of MouseButton1Click and Activated makes it compatible across all devices.

Accept this answer if it helped.