On the OM13077 Development kit, the ISP Pin on the SWD Debug Port is missing. The SWD Header has no pin on position 7!!!
Found this issue while trying to debug / flash a LPC54120 which wasn't responding.
I tried the solutions from the FAQ but no one was working. Then I checked the ISP connection to my board and noticed, that the ISP pin was never pulled low.
I had to connect the ISP (pin 7) manually to my target board.
Thank you NXP, I had some fun debugging, why my target doesn't want to respond. (There was no single note for this issue)