The line that the error pops up :
manager.getUserInventoryContents('76561198083528564', 440, 2, true, function (error, inventory) {
console.log(inventory.length);
for(let l = 0 ; l < inventory.length; l++) {
if(inventory[l].market_hash_name == prices[i].name) itemid = inventory[l].assetid;
}
})
Error: