How to Get Random Numbers NOT Equal Parts?

First, you should indent your code correctly. It makes it much easier to read, and therefore much easier to catch mistakes. Likewise, you should generally use spaces whenever possible to split things up visually, such as around operators like (<) and (>).

while true do
    wait(1)
    local type = math.random(1,10)
    if type < 7 then
        print("grass")
    elseif type > 6 < 9 then
        print("road")
    elseif type > 8 < 11 then
        print("water")
    end
end

Although it isn't related to the error you're having, you should avoid defining a variable named type. That's because type is a function that's built-in to Lua which tells you the type of a parameter (type(4) is "number" and type("hello!") is "string").

The main source of your error, though, it exactly where it's telling you. On line 7, you're attempting to compare a number with a boolean.

Why?

We have this expression to evaluate: type > 6 < 9. Okay, let's pretend that we say type = 7 just before this. How does Lua understand this?

type = 7 -- define type to be 7

So we want to evaluate this:

type > 6 < 9 

Let's plug in type's value:

7 > 6 < 9

As Lua understand this code, there are parentheses there. Even though you didn't write them, they're implied by the parsing rules. I'll show them to you:

(7 > 6) < 9

Okay, great. So let's evaluate what's inside the parentheses.

Since 7 > 6 is obviously true, we get (true).

(true) < 9

And now drop those pointless parentheses:

true < 9

But what is this supposed to mean? I don't know whether true is more or less than 9. Lua doesn't either, that's why it's crashing.

What you actually want to say is:

type > 6 and type < 9

The and operator (it's not a variable) takes something at its left and its right. If they're both true, then the result is true. Otherwise it's false. (Technically it is more complicated than this. Until you understand the above, however, ignore this bit: The left argument is evaluated; if it is falsey, it is returned, otherwise the right argument is evaluated and returned).

So we can fix your code by writing:

while true do
    wait(1)
    local type = math.random(1,10)
    if type < 7 then
        print("grass")
    elseif type > 6 and type < 9 then
        print("road")
    elseif type > 8 and type < 11 then
        print("water")
    end
end

Let's address your error first.

On line 7 you tried to do an

and statement, without putting the and. We can fix this easily by chanign line 7 to:

elseif type > 6 and type < 9 then

Easy, common mistake. Now let's look at your question and find a solution.

Tables.

I have yet to write a good answer on tables, so I figure I can do that while still answering your question.

Tables are extremely great tools, mainly in situations like this. Let's set up 3 tables to use, which would mean 3 classification levels. Our levels will be;

• Common

• Moderate

• Extremely Rare

Let's setup the tables:

common = {"Common stuff here.."}
Moderate = {"Moderate 1", "Moderate 2", "etc..."}
Rare = {"asdasd","asdasd"}

Great. Now let's figure out how we can randomize it.

You are on the right track with using math.random, but to make it even more rare, we are going to use math.random, and make our maximum random up to 100.

Let's try it out:

path = game.Lighting["Stuff"] -- The model/file/place in which all the things you want to spawn are in.
toSpawn = game.Workspace -- Where the thing that's cloned would go into. 

function spawn()
    local random = math.random(1, 100)
    if random <= 50 then
        local RandomInTable = math.random(1, #common)
        local part = path:FindFirstChild(common[RandomInTable])
        local partClone = part:Clone()
        partClone.Parent = toSpawn
    elseif random > 50 and random <= 80 then
        local RandomInTable = math.random(1, #common)
        local part = path:FindFirstChild(Moderate[RandomInTable])
        local partClone = part:Clone()
        partClone.Parent = toSpawn
    else -- Now we can just use else, no other situations are possible.
        local RandomInTable = math.random(1, #common)
        local part = path:FindFirstChild(Rare[RandomInTable])
        local partClone = part:Clone()
        partClone.Parent = toSpawn
end

Now, with that, you would just need to call the function spawn.

Finally

Hopefully I helped you even a little bit. Even if I just pushed you into your own thought. Good luck to your development, and feel free to leave any questions in the comments about my explanation.