Developer Product is not adding 1 to the Value?

The problem is that you are using a LocalScript for handling the purchase, where you should be using a normal Script.

What you need to do is put the code into a normal script, get rid of the player and stage variables and then use the player found in the for loop (reference by the variable 'v' without quotes) to increase their stage value.

Also, you need to return PurchaseGranted inside your purchase handling function, otherwise you will not receive any money from ROBLOX.

Overall, your final script should look like this:

local MarketplaceService = game:GetService("MarketplaceService")
local ds = game:GetService("DataStoreService"):GetDataStore("PurchaseHistory")

CASHID = 24397415 -- Put the Developer Product ID here

MarketplaceService.ProcessReceipt = function(receiptInfo)
    local playerProductKey = "player_" .. receiptInfo.PlayerId .. "_product_" .. receiptInfo.ProductId
    local numberBought = ds:IncrementAsync(playerProductKey, 1)
    for i,v in pairs(game.Players:GetPlayers()) do
        if v.userId == receiptInfo.PlayerId then
            if receiptInfo.ProductId == CASHID then
				local stats = v:WaitForChild("leaderstats") --Yields the script until leaderstats becomes available.
				local stage = stats:WaitForChild("Stage") --Same as above, but waits for the stage value in leaderstats.
				local char = v.Character --Referencing the character.
				if stage.Value < 25 then
    				stage.Value = stage.Value + 1
    				--No need for the wait. The script will wait if the character's humanoid doesn't exist yet.
					char:WaitForChild("Humanoid").Health = 0 --Waits for the humanoid then kills it by setting it's health to 0.
				end
			end
		end
	end
	return Enum.ProductPurchaseDecision.PurchaseGranted --You need to return PurchaseGranted in your function, otherwise you will not recieve any money from a transaction.
end

I hope my answer helped you. If it did, be sure to accept it.