Board Thread:Tapped Out Help/@comment-26890237-20170212081855/@comment-26143371-20170212224202

In addition to the level requirement, the game also requires you to complete the main questlines from all the previous levels. So make sure that you have completed Executive Lackey and the quests before that.

In general you do level up faster than you can complete the associated quests. Based on that I would guess that you're doing level 22 or 23 quests right now.