1: Steam Guard is not launched
In order to protect your items and money in Steam account from embezzling by people who get your password illegally, Steam requires you to launch Steam Guard for more than 15 days. If you have not launched it for 15 days, you will be unable to trade or use Steam market. Also, accounts which forbidden Steam Guard are unable to trade.
2: You have reset password of Steam account or changed bound mailbox
If you have reset your password or changed bound mailbox, you will be bidden to trade in 7 days. If your account does not take any action in 2 months, you will be bidden to trade in 30 days.
3: The condition of your Steam account’s information or inventory is set as privacy
Your condition of your Steam account’s information or inventory is set as privacy so it causes that robots cannot send offers to you.
4:Wrong trade URL
Because of wrong trade URL, robots cannot send offers to you. Format: https://steamcommunity.com/tradeoffer/new/?partner=（steam id）&token=XXX（random value）
5: Steam account is forbidden
Because you violated rules of Steam market, so your account is forbidden. According to severity, you may receive a temporary or perpetual trade prohibition. To solve the problem:
5.1: How to launch Steam Guard
If you are a Steam user:
a: Enter into home page of Steam client, click “Steam” on the top left corner, select “Settings”.
b: Check your security status, only accounts which are protected by Steam Guard for more than 15 days can be allowed to trade and use market function.
5.2: How to set privacy of inventory as open and bound
a: Go to Items Storage
b: Click the place where the arrow points at (Click here to check your Steam trade URL)
c:Enter into your Steam account, “Who can send offers to me” page, copy the URL link. Then enter into your Steam account again, “Private settings” page.
d: Check if your information and inventory are open.
5.3: How to write trade URL
a: Paste the copied link to place where the arrow points at and save.
b: If the link loses, you can click the place where the arrow points at (Click here to check your Steam trade URL).
c: Enter into “Steam trade offers” page and set, completely copy the trade URL.
d: Paste and save.