Why am I getting this error?
Most modern PCs and laptops support two boot modes: UEFI (EFI) and Legacy (BIOS). When you create a bootable USB stick, you choose in which mode it should be created.
If you boot into Legacy mode from a USB flash drive recorded in UEFI mode, you will see the message “ERROR: BIOS / LEGACY BOOT OF UEFI-ONLY MEDIA”, and the explanation on the error screen suggests either changing the boot mode in the BIOS/UEFI settings of your computer , or overwrite the USB flash drive for a different boot mode.
Switch from Legacy to UEFI
I want to note right away that not all computers and laptops have such an opportunity. Older computers (2010-2012) may not support UEFI mode, which means that they can only boot in Legacy mode. Therefore, if you know that your computer or laptop doesn’t support UEFI, go straight to the second method of fixing the error.
To switch boot modes, you need to go into the BIOS.
Next, go to the "Boot" tab (the names may differ depending on the manufacturer and firmware version).
Find the option “Boot List Option” or "UEFI / BIOS Boot Mode” and change the value in it to UEFI.
Create Legacy Bootable USB Drive
If for some reason the previous solution is not suitable for your case, you can recreate the bootable USB flash drive so that it is prepared for Legacy mode. To do this, you can use the free utility Rufus.
To do this, in the section "Partition scheme" select MBR, and in the "Target system", select "BIOS (or UEFI-CSM)". After that we press "Start" and wait for the end of the process. Next, try to boot from the newly created bootable USB flash drive. Now there will be no error.