Why is my script editing the same part instead of creating a new one?

The problem is that the part is created outside the loop so you are just editing its properties instead of creating a new part which is what you need.

This Problem

local sp = script.Parent
local PartsToGenerate = 10

function BlastAttack()

    local Part = Instance.new("Part") -- part is created here
    Part.Anchored = true

    for Gen = 1, PartsToGenerate do -- you loop to make a circle but you do no create a new part
        Part.Parent = workspace
        Part.CFrame = CFrame.Angles (0, math.rad(360/PartsToGenerate*Gen), 0) -- why do 360/10? or why do this each time its the same math porblem
        wait()
    end

end

while wait(1)do
    BlastAttack()
end

The fix with some additions

local sp = script.Parent
local PartsToGenerate = 10

local num = 360/PartsToGenerate -- do some math here to save time in the loop

function BlastAttack()

    for Gen = 1, PartsToGenerate do 
	-- this now create a part each time the loop is ran 
	 local Part = Instance.new("Part",  game.Workspace)  -- second arg is optional, which is the pars parent
  	Part.Anchored = true
        Part.CFrame = CFrame.Angles (0, math.rad(num *Gen), 0) -- simpler math =  more efficient 
        wait()
    end

end

while wait(1)do
    BlastAttack()
end

Hope this helps.