For trading on Steam, your Steam account needs to fit various requirements. The most common problems are:
You can not bypass these restrictions. Which means, in worst case you need to wait up to 15 days to be able to trade on Steam again.
There are multiple ways to find out why your Steam account is currently unable to trade. The best way is to send a trade offer to a random Steam friend. It will abort and show you the reason why you are unable to trade.