Board Thread:Tapped Out Help/@comment-216.15.114.122-20150508230320/@comment-26143371-20150509123603

In order to start the quest of a specific level(i.e. level 21) you need to have completed or at least started the quests from the previous level(i.e. level 20). Additionally there might be some other dependencies, which are not displayed.

A general rule here is: Just keep doing the quests you have. At some point things will unlock.