Ran into error: attempt to index nil with 'FindFirstChildWhichIsA', any fixes?

I think your problem is you are trying to define player by LocalPlayer inside a server script, which wont work. game.Players.LocalPlayer is meant to define the local player on a client, which the server doesn't have access to. Instead use a server script and loop through all the players every 60 seconds adding cash to each one

while true do -- Loop
	wait(60) -- Wait 60 seconds
	for _,Player in pairs(game.Players:GetChildren()) do -- Iterate through each player, running below code on each one
		local lstats = Player:FindFirstChild("leaderstats") -- Find leaderstats
		if lstats ~= nil then  -- If leaderstats found then
			lstats.Money.Value = lstats.Money.Value + 50 -- Add some money
		end
	end
end

Here are some ways to get the player via server scripts

game.Players.PlayerAdded:Connect(function(player) -- This function will run for each player when they join the server, you can then run some code like below
print(player)
end


--You can get a player from remote events fired from a local script
local function TestFunction(player)
print(player)
end
game.ReplicatedStorage.Remotes.TestRemote.OnServerConnect(TestFunction)

-- You can get players from click detectors
function onMouseClick(player)
	print(player.. " clicked me!")
end
 
part.ClickDetector.MouseClick:Connect(onMouseClick)