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!

You Fixed Most Of My Problems Through Comments, But I come across this error.

19:47:33.978 - ServerScriptService.New Server:2: 'then' expected near 'in'

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

This code seems to be broken, Also I don't understand what u mean by if '.' in, what am i supposed to fill in there?