Hi
This may be a false message. The actual issue is that Debian is not a supported GOS for Converter. Hence it treats it as 'Other Linux' and then the ESXi responds that it does not support 'other Linux' on EFI.
Unfortunately I can't think of any meaningful workaround. There is a wild idea - Converter checks whether the source machine is BIOS or EFI by looking for /sys/firmware/efi/vars directory. If you can somehow fool it that it does not exist (I don't know how), it would treat the source as BIOS and convert it. In case that happens you should then change the destination VM firmware to EFI and manually create the EFI boot entry somehow.
Regards
Plamen