Sparse information from HP indicates that the
"ROM checksum error"
is not the same as the
CMOS checksum error.
To fix that "ROM checksum error"m one can:
1. Replace the motherboard.
2. Try to boot a floppy with the ROM recovery program
3. Some other repair or recovery method not well documented.*
I did not find the exact information on the HP site. Other sources say that the "ROM checksum error" waits for the floppy drive. The floppy is to have the recovery program for that BIOS chip.
I had an I Intel motherboard a few years ago with that feature. But I don't know if the OP 'motherboard has that feature. It is not something you often run into.
* There is a gizmo to program the BIOS with no power to the motherboard. Without removing the chip.