If you can't find a workout to Plan or log it's probably because it hasn't been added to the site yet. That means it's time to build it. After you build a workout it will be available for other BTWB users to find and use.
When in doubt about the workout already existing on the site or not, 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.