OnServerInvoke is not a valid member of RemoteEvent.How to fix?

Tried using:

game.ReplicatedStorage.tryToPurchase.OnServerInvoke:connect ( function (player, cost , spaces)

or

game.ReplicatedStorage.tryToPurchase.OnServerInvoke = function(player, cost, spaces)

but i get the same error.

LocalScript: (GUI)

local buyButton = script.Parent
local newSpaces = 1000000
local upgradeCost = 100000000

local function giveUpgrade()
    game.ReplicatedStorage.tryToPurchase:Invoke(upgradeCost, newSpaces)
end

script.Parent.Activated:Connect(giveUpgrade)

Script: (ServerScriptStorage)

game.ReplicatedStorage.tryToPurchase.OnServerInvoke:connect(function(player, cost, spaces)

    local playerStats = player:FindFirstChild("leaderstats")
    local playerGold = playerStats:FindFirstChild("Gold")
    local playerSpaces = playerStats:FindFirstChild("Spaces")

    if playerGold.Value >= upgradeCost then
        playerGold.Value = playerGold.Value - cost
        playerSpaces.Value = spaces
    end
end)