If you can't find a workout to Plan or log, it's probably because it hasn't been added to CrossFit btwb...yet. That means it's time to build it. After you build a workout it will be available for other users to find and use.
If you aren't sure whether the workout already exists on the site, build it anyways. The site will not create a duplicate workout. Instead, it will find and pull the existing workout for you to use. For example, you will be unable to create a new Fran workout. You can try, but assuming it's an identical version to the existing Fran, you'll simply be given that one to Plan or log a result for.