I need help on making a place block tool? [Need Snapping Fix]

I actually achieved the result you are looking for recently so I can help you. First off, the blocks can go in Replicated storage. Next, for a Minecraft type game, you should make a folder in the workspace called "Blocks" and have a server script in ServerScriptService do the following

game.Players.PlayerAdded:Connect(function(plr)
	Instance.new('Folder', workspace.Blocks).Name = plr.Name
end)

Then you can move all the blocks a player places into their folder which may come in handy later. You should get the mouse coords and everything and send it to a server script in ServerScriptService. If the block is placed from a LocalScript only the player who placed it will see the block. Also, if a localscript does all the math, it can be exploited by hackers. Here's a example of what your LocalScript might look like. I put a RemoteEvent in ReplicatedStorage called PlaceBlock. I also made another event called RemovePassive that will be used to remove passive blocks.

local mouse = game.Players.LocalPlayer:GetMouse()
local OriginalMousePosition = mouse.Hit.p
local Active = false
local tool = script.Parent

tool.Equipped:Connect(function()
	Active = true
end)

tool.Unequipped:Connect(function()
	game.ReplicatedStorage.RemovePassive:FireServer()
	Active = false
end)

mouse.Button1Down:Connect(function()
	if Active then
		game.ReplicatedStorage.PlaceBlock:FireServer(mouse.Hit.p, false) --Bool value represents passive bool
	end
end)

while wait() do
	if Active then
		if OrginalMousePosition ~= mouse.Hit.p then
			OrginalMousePosition = mouse.Hit.p
			game.ReplicatedStorage.RemovePassive:FireServer()
			game.ReplicatedStorage.PlaceBlock:FireServer(mouse.Hit.p, true) --Bool value represents passive bool
		end
	end
end

Here's what the server script in ServerScriptService would look like

local function IsInt(number) do
	if '.' in tostring(number) then return false else return true
end

local function NearestMultiple(number, multiple) --I think this function works
	number = math.floor(number)
	local newNum = number
	local count = 0
	while not IsInt(testNum/multiple) do
		newNum = testNum - 1
		count = count + 1
	end
	if count <= multiple/2 then
		return newNum
	else
		return newNum + multiple
end

game.ReplicatedStorage.PlaceBlock.OnServerEvent:Connect(function(plr, pos, passive)
	BlockName = --Get name of block
	local block = game.ReplicatedStorage.Blocks[BlockName]:Clone() --A folder called Blocks in ReplicatedStorage that contains all the blocks
	local BlockSize = block.Size.X --I can grab one value if all the values of the size are the same
	local x = NearestMultiple(pos.X, BlockSize) --Rounding the positions will give it a more Minecraft aspect
	local y = NearestMultiple(pos.Y, BlockSize)
	local z = NearestMultiple(pos.Z, BlockSize)
	block.Position = Vector3.new(x + _, y + _, z + _) --I'll leave you to fill in the blank. You have to add something so it centers rather than half being in the ground.
	if passive then
		block.Transparency = 0.5
		block.CanCollide = true
		block.Name = 'Passive'
	end
	block.Parent = workspace.Blocks[plr.Name]
end)

It should look something like this You can do the RemovePassive script, but let me tell you to use a "pcall(function()" in it, for when it is fired without there being a passive block. Hope this helps!