EliteOneTUbe

That's not necessarily what that means. What exactly that error means is pretty much unknown. The takeaway here should be that you need to catch and handle errors, or else your stuff will crash.

I guess its time for me to google how to handle errors

That's just Steam being Steam.

So I cant call my inventory if I called it a few seconds ago?

Check error.

{ Error: The request is a duplicate and the action has already occurred in the past, ignored this time
    at C:\Users\Nick\Desktop\botv2\node_modules\steamcommunity\components\users.js:440:12
    at SteamCommunity._checkHttpError (C:\Users\Nick\Desktop\botv2\node_modules\steamcommunity\components\http.js:110:3)    at Request._callback (C:\Users\Nick\Desktop\botv2\node_modules\steamcommunity\components\http.js:50:61)
    at Request.self.callback (C:\Users\Nick\Desktop\botv2\node_modules\request\request.js:185:22)
    at Request.emit (events.js:182:13)
    at Request.<anonymous> (C:\Users\Nick\Desktop\botv2\node_modules\request\request.js:1161:10)
    at Request.emit (events.js:182:13)
    at Gunzip.<anonymous> (C:\Users\Nick\Desktop\botv2\node_modules\request\request.js:1083:12)
    at Object.onceWrapper (events.js:273:13)
    at Gunzip.emit (events.js:187:15) eresult: '29' }

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: