PXE Boot Stuck on "Waiting for Approval"

This can be quite a few things;  DHCP scope, IP Helper, and boundry group setup in SCCM to name a few.

A less known issue can be if you have recently implemented NSX in vCenter.  Turns out that the install with nothing configured is not actually in a "listening only" mode.  

The default Segment Security Policy in NSX has DHCP Server Block set to Enabled!  This blocks the DHCP handshake with SCCM during PXE.  You need to create a new Segment Security Policy, set DHCP Server Block to disable and apply that policy to your SCCM server's VLAN under Distributed Port Groups.