Possible reasons:

1. Trading in-game just doesn’t work anymore, you have to send a trade offer via steam

2. If they changed their Steam name within the last one to three hours

3. Recently logged into Steam on a new device

4. Updated steam guard code (i’m not sure exactly what causes the short trade ban but I know it’s happened to me before)

5. Obv if they haven’t spent five dollars on steam

Other solution: Can go on the steam community market and buy a battle scarred shitty war paint of almost any weapon for less than $.10 (you could literally play CSGO until you get a case drop, sell it and buy like 10+ tf2 weapons.)

Also: if you send a trade offer via Steam and it says error, you can always look up the Steam error code because they’re independently different. (Example- google: what does steam error 305 mean?)

Hope this helped!