i got the same issue than you.
I have an HP 6000 Pro as main Linux PC.
I tried to install Batocera on a HP dc7900. I've burned the img with etcher on USB Stick but not enable to boot on it.
I tested the USB stick on Laptop Lenovo, and old EEpc with success.
I tried to with Lakka & Recalbox and i got the same issue.
I updated the Bios to version 1.27 Rev. A. and the ME Firmware to 126.96.36.1999 Rev. B.
Still the issue, and i got a flashback from my windows years when i was trying to boot an a specifique disque, on windows we need to mark the Partition Bootable.
So in Xubuntu, with gnome-disk-utility i set the partition bootable.
And i tried with the USB Rear panel, seems for me that the USB Front Panel do not work very well.
Next Step, try to install on SSD.