MEL

What does "APU was placed on MEL" mean?

Minimum Equipment List

A system on the APU has failed or the complete APU is inoperative. 

The aircraft is legal to fly as long as the system has been deferred under the proper documentation and procedures.

In the case of the APU. Two working generators on the engines. Two working bleed air systems,  also from the engines. 

The main engines will require an external air start.