In order to trade in Steam, you should have your mobile authenticator installed for more than 15 days. You should check if your mobile authenticator is active right now. If it is not active, you should activate it using this guide: How to activate Steam Guard mobile authenticator?
To remove the restriction, 15 days must pass since the first activation of the mobile authenticator. You are able to get more information about the restrictions on Steam support page: https://store.steampowered.com/phone/manage
If it is active, you should just wait for 15 days if you set up it for the first time or 7 days in case you reinstalled Steam Guard.
You can always check if you can trade without Steam trade hold by clicking on this link and adding your item to the trade. If your trade will be held, you will see this message from Steam.