Upgraded iMac iFixit SSD now won't boot up (one LED only)

Hi there,

My 2019 27" 5k iMac stopped booting into macOS and only intermittently detected the internal drive when I booted into recovery mode and ran disk utility.

I think I know the answer here but I attempted to fix this issue by removing the blade SSD from the logic board and installing a 2.5" SSD in the HD mount (via an Apple SATA cable) from iFixit's SSD upgrade kit (I followed the directions for a blade SSD upgrade here on iFixit).

Everything seemed to go well.

However after reassembly, the mac won't start at all (no chime, only the first diagnostic LED lit up on the logic board). I've tried shorting the power button connector and also confirmed I'm getting 12v over the ribbon connector between the power board and logic board and also on the pins where the main power comes into the logic board and I'm getting voltage. I don't see anything else obvious.

Do I need to have something installed in the onboard blade SSD slot? Or is there something I should have done to change to the mac to use / boot from the 2.5" SATA SSD?

I'm guessing the real answer is I've somehow damaged the logic board in the disassembly / reassembly process (I was pretty careful and didn't have any obvious mistakes)?

Thanks for any thoughts or advice!

Dave.