How to create a bootable flash drive to boot it on a computer without uefi?

Question

UEFI

There was a bootable USB flash drive with Ubuntu Server, I bought an old PC specifically for testing servers, I wanted to install it, an error came up: "SYSLINUX: No DEFAULT or UI configuration directive found" I decided to overwrite the image on the USB flash drive and after that it stopped booting from it at all, although in the boot screen JetFlash is present, I tried to format it under FAT / NTFS and install it through various programs (Universal USB Installer, Unetbooting, rufus) no reaction. What I noticed is that earlier when installing the same Linux on the main one (Modern PC with uefi) in UEFI, the flash drive was displayed with 2 icons, just JetFlash and marked UEFI, and now only UEFI, so I think that maybe it is recorded, only for UEFI and therefore the ancient BIOS does not see it?
PS Sorry if it's written too amateurishly, the first time.
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UPD. Solved with Rufus by ticking "Add fixes for older BIOSes"

Answer 1

Butler.
That says it all.

Answer 2

Maybe someone will need a solution for Mac: https://etcher.io/ most likely works under Linux too. Analogues like UNetbootin do not work for old laptops where there is only BIOS mode!