"If no partition table exists, the ArcaOS installer will automatically create the
table as selected in the Installation Volume Manager during installation [...]"
So, good to know.
Perhaps they're using the DFSee runtime to handle the GPT layout (pure speculation).
FWIW... 'x:\sys\install\EspUtil.exe' is the pgm that formats and populates the ESP. It has plenty of options for DIY'ers but nothing I saw to do repartitioning.
Checking the BIOS boot order I found AN Launcher and selected the install partition, boot started and stopped fairly quickly showing os2lvm.dmd on screen and the "OS/2 cannot operate your hard disk..." message.
Not sure where to go from there... all partitions look good in miniLVM, chkdsk does not report any problems.
If the system can't match the boot drive to a letter, you'll get that error. So, just to confirm,,, when booted into the installer and running MiniLVM, does the target (install) drive have the expected letter? Does that match the boot drive referenced in the target's 'config.sys'?
Next, is everything (ESP and boot partition) on the same drive? When split across drives there's a chance that the disk numbering is changing during bootup. Ensure that the machine considers the disk with the ESP to be Drive 1 (i.e. the first in its list of hard drives).
BTW... having the bootup hang after the first reboot, just as it's about to go graphical, has happened to me many times. This could just be voodoo, but in at least some cases the target drive's 'os2.ini' was still marked read-only. Resetting that flag then rebooting seemed to get the install going again. Take this with a grain of salt...