HP Z420 Slot 2 (PCIe 3x16) not working with NVS 450 or RX 470/560

Hello, I have a HP Z420v1 with a 600W PSU and it originally came with a NVIDIA Quadro NVS 450, which I replaced with a RX 470. When I put in the RX 470 and plugged in the necessary PCIe connector (one 6-pin), it POSTed into Windows 10 but then suddenly I got 6 beeps and a red flashing power button, and then it turned off. I tried with a RX 560 but the same thing happened, except this time it didn't POST and instead started beeping 6 times. Now when I try with a NVS 450 it refuses to POST, even after resetting the CMOS. The bottom slot (Slot 5) that is wired as PCIe 3.0 x16 does work however with the NVS 450, but the RX560 and RX 470 don't fit in there. I would soon like to put in a RTX 3060 in here so if this doesn't get fixed its a big problem.