Make sure you have accepted the Steam Subscriber Agreement. You can find it by going to the workshop front page, then click the link to your workshop (on the right), then it is on the right under your name.

If you haven't spend any money on that account, it could be a limited user account. That would explain the restriction.https://support.steampowered.com/kb_article.php?ref=3330-IAGK-7663