This might happen when we are experiencing issues with the steam api, making all tradeoffer urls seem faulty to us. Otherwise it might be because you have done something which triggered SteamGuard's protection. Things like changing your email, changing your password, logging in from a new device or browser, logging in from a browser after clearing your cache and cookies, changing your authenticator device, and other things of the sort trigger so.

You will have to wait up to two weeks in order to be able to trade again, you can find more information regarding this topic in the following Steam Support article https://support.steampowered.com/kb_article.php?ref=1047-edfm-2932