I'm not familiar with the site Esperanto.com and can't speak for its general reliability. Airvian is correct that the O is optional in compounds and the decision whether to include it usually comes down to how easy it is to pronounce without it. In this case, you probably would leave it out. That is ***labortablo*** is more common.

In turn, ***stablo*** is more common than *labortablo* - but the course rejects this answer.