If you’re certain the Challenge you wish to enter is still open for submissions, but it doesn’t appear as an option to submit your entry, chances are that particular Challenge is exclusive to Vocal+ members, and you have a Vocal Free account.
To be sure, you can visit the home page of the Challenge by visiting this page and selecting the Challenge you’re interested in. If there’s a pink button that reads “Upgrade to Vocal+ to enter” on the page, and if the page states in the “About” section that it’s open to Vocal+ members only, then it’s not open to you because you don't have a Vocal+ account.
If you still wish to enter that particular Challenge, you can learn more about Vocal+ and upgrade the account here. Otherwise, we always have several Challenges running at once, and are constantly launching more all the time—including those open to free users as well. You’re sure to find one you’d like to enter, even if you don’t want to upgrade right now.