This is a common issue that you might experience a lot due to some Steam issues and other reasons.
"There was an error accepting this trade offer. Please try again later. (ERROR NUMBER #)"
#8, 50, or 24: Your Steam trading session expired, you need to refresh the trade offer after logging out and back in of Steam to fix it.
#11, 16 or 42: The trade went through, you only need to refresh the page to see that it's no longer valid or check your inventory/backpack. Sometimes when you experience the errors 15, 16 or 25 and the trade didn't go through, you have to restart Steam and try to accept it again.
#15, 25 or 26: Your or the bot's backpack is full. Free some space in your backpack or use another bot that isn't full. It also can mean that the items were already traded away.
#2, 20 or 28 : Steam is experiencing some item server issues and you will have to try again later.