-- start with this:
local InventoryAmmo = (player.inventory[player.equipment.ammo].count or 0)

-- break it up into this:
local inventory = player.inventory
local equipment = player.equipement
local ammo = equipment.ammo
local temp = inventory[ammo]
local count = temp.count
local InventoryAmmo = count or 0

-- the error happens at count = temp.count, so temp must be nil. fix it by adding "or {}"
local inventory = player.inventory
local equipment = player.equipement
local ammo = equipment.ammo
local temp = inventory[ammo] or {}
local count = temp.count
local InventoryAmmo = count or 0

-- replace count:
local InventoryAmmo = temp.count or 0

-- replace temp, in order to make it work you have to add parens:
local InventoryAmmo = (inventory[ammo] or {}).count or 0

-- and then the rest is easy:
local InventoryAmmo = (player.inventory[player.equipement.ammo] or {}).count or 0