How to solve the problem of "No relief from work! Leaving!"?

First I have to say, The Megatowers in general have probably the most bugs known to this game as of right now. You need to be very careful on what is going around your city as you are building apartments in your Megatower.

Generally when they are saying they are overworked...

"Overworked" means they worked on both shifts of the day for extended period of time. There are day shifts and night shifts. Unlike normal residential RCI buildings, residents in megatowers can work on both shifts, and when this continues they will complain and leave.

The reason they work on both shifts is because there is no other free workers available. To solve this, you need more apartments or zone more residents, and make sure the road layout is sufficiently optimized so that they won't miss their shifts.

You should also try to build a level in your Megatower that has a park OR Use the crown as a park on the top level. The more likely issue is that the tower is just bugged. Demolish that level of the building, and rebuild it.