I had the same issue, for me updating the TPM firmware solved the issue (but it is convoluted)
https://superuser.com/questions/1668861/alert-tpm-device-is-...
My guess is that Dell didn't update the TPM firware, my guess is this breaks some keys so can't be done automatically and a BIOS update at some point then screws up the handshake with the older TPM firmware unless the laptop is fully powered off.
But I'm not a huge fan of dell hardware.
I don't want to go back to OS X, but its hard to find good build quality and a high res screen.
Also, my battery was terrible after a couple of years. I had it plugged in as well. I bought a replacement this year and it was easy enough to switch out. Hopefully I can just keep this going and use it as an RDP machine.
[1] https://www.reddit.com/r/Dell/comments/onvh42/xps_9560_tpm_d...