What's going on?

You're trying to get a number of time of day, which is a string. When you use tonumber you turn the whole string into a number, but if the string, like time of day has ":" in them, it returns nil, because it isn't a number, and you cannot compare nil with a number.

How to fix it

You need to find a way to cancel out the ":" from the text. It looks like you only need the first 2 characters because you're only finding the hour. This can be solved with string.sub() .

How does it fix my code?

Like I said, the problem is the string with non-numerical characters in it becoming numbers, which returns nil. If we cut out a section of the string to only say the numbers, it won't return nil. For example:

print( string.sub("Hi Mom!", 1, 4) )

Will print out:

Hi M

Final Code

while true do --:sub() is a shorter way to write string.sub().

    while tonumber(game.Lighting.TimeOfDay:sub(1,2)) < 8 or tonumber(game.Lighting.TimeOfDay:sub(1,2)) > 20 do
        wait(1)     
        script.Parent.PointLight.Brightness = 15
        script.Parent.Material = "Neon"
    end

    while tonumber(game.Lighting.TimeOfDay:sub(1,2)) > 8 and tonumber(game.Lighting.TimeOfDay:sub(1,2)) < 20 do
        wait(1)     
        script.Parent.PointLight.Brightness = 0
        script.Parent.Material = "SmoothPlastic"
    end
end

How can I stop making the same problem?

Check and be aware that non numerical characters like the ones in the time of day, will return nil when turned into a number, which is normal. The error is when you try to see if the "nil" is greater than or less than a number, which will error. Try to watch out for those and check other string manipulation functions to work with later if you get the same problem.

Hope it helps!