I had the same error with today's ubuntu-20.10-desktop-amd64.iso image on a Shuttle SG45H7 system with an Intel Core2 Duo E4400:

  GRUB Error: can't find command "grub_platform"

Which then leads to a blinking cursor black screen after selecting any GRUB entry.

An alternative solution that works is to flash the ISO to the USB drive with Startup Disk Creator then convert it to MBR.

# sudo fdisk -l /dev/sdb
GPT PMBR size mismatch (5746099 != 123404287) will be corrected by write.
The backup GPT table is not on the end of the device.
Disk /dev/sdb: 58,84 GiB, 63182995456 bytes, 123404288 sectors
Disk model: Transcend 64GB
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: gpt
Disk identifier: 7EE1FFAC-4072-46B8-885E-A7EA3F9C70CF

Device       Start     End Sectors  Size Type
/dev/sdb1       64 5735483 5735420  2,7G Microsoft basic data
/dev/sdb2  5735484 5745435    9952  4,9M EFI System
/dev/sdb3  5745436 5746035     600  300K Microsoft basic data


# sudo gdisk /dev/sdb
GPT fdisk (gdisk) version 1.0.5

Partition table scan:
  MBR: protective
  BSD: not present
  APM: not present
  GPT: present

Found valid GPT with protective MBR; using GPT.

Command (? for help): r

Recovery/transformation command (? for help): ?
b       use backup GPT header (rebuilding main)
c       load backup partition table from disk (rebuilding main)
d       use main GPT header (rebuilding backup)
e       load main partition table from disk (rebuilding backup)
f       load MBR and build fresh GPT from it
g       convert GPT into MBR and exit
h       make hybrid MBR
i       show detailed information on a partition
l       load partition data from a backup file
m       return to main menu
o       print protective MBR data
p       print the partition table
q       quit without saving changes
t       transform BSD disklabel partition
v       verify disk
w       write table to disk and exit
x       extra functionality (experts only)
?       print this menu

Recovery/transformation command (? for help): g

MBR command (? for help): ?
a       toggle the active/boot flag
c       recompute all CHS values
l       set partition as logical
o       omit partition
p       print the MBR partition table
q       quit without saving changes
r       set partition as primary
s       sort MBR partitions
t       change partition type code
w       write the MBR partition table to disk and exit

MBR command (? for help): w

Converted 3 partitions. Finalize and exit? (Y/N): y
GPT data structures destroyed! You may now partition the disk using fdisk or
other utilities.

# sudo fdisk -l /dev/sdb
Disk /dev/sdb: 58,84 GiB, 63182995456 bytes, 123404288 sectors
Disk model: Transcend 64GB
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0x00000000

Device     Boot   Start     End Sectors  Size Id Type
/dev/sdb1            64 5735483 5735420  2,7G  7 HPFS/NTFS/exFAT
/dev/sdb2       5735484 5745435    9952  4,9M ef EFI (FAT-12/16/32)
/dev/sdb3       5745436 5746035     600  300K  7 HPFS/NTFS/exFAT


PS: thanks @iaincc for the GPT => MBR tip!