why I can't load my iso file in virtualbox?

I have one iso of exactly 512 bytes that only contain a master boot record and I can't load it in virtualbox. It says "Could not get the storage format of the medium 'C:\Users\name\Desktop\info\e.iso' (VERR_NOT_SUPPORTED).". The thing is that I just want to use that iso as a disk that can be loaded in virtualbox by the bios. I don't want anything else. I don't understand why I couldn't use it even if it would be corrupted.

ISO is a CD file format, not a raw disk format. Any program and/or BIOS would expect to find a particular format.

You having written an MBR boot sector to a file, then renamed that file as .iso does not make it a valid ISO file that is bootable.

What I would do to achieve what you want is to create a new virtual disk in VirtualBox, boot from a real Linux Live ISO image, and then use dd to write your MBR bootsector to your virtual disk.

The disk may well be more than 512 bytes, but the disk image will be valid both inside the VM, where it will be a "real disk", and outside the VM where it will be a real "disk image".

Unix Stack Exchange: Using dd in order to save and restore a boot sector

Alternatively if you really want a bootable ISO file, you still have to create it properly using tools that understand the iso format

From Creating a bootable CD with custom boot sector you probably want to use the command

mkisofs -R -b path/to/loader.sys -no-emul-boot -boot-load-size 4 -o [IsoFile.iso] [IsoDirectory]