As per below link Nvidia A16 supported only on Riser 1A+2E+3B. Not sure system is not booting because of this. Which slot did you connected GPU? [URL]   
Hi all,  i had issue with this server and GPU, i bought risers/cab :  risers PN:  DTTHJ,J7W3k,MDDTD all A risers. Cables for GPU : TR5TP (i can only add 6 pin) is that ok ?  2xCMPGM (power supply)    When i add everything inside the server, server wont boot, and power supplies blinking amber-green all the time. Does anyone know this is issue.   This is urgent to solve i lose a lot of time because of this .   Thank you all for help .      
PER740 GPU Dell NVIDIA A16 Passive GPU part2 .