I am currently running Debian 11, and I have another partition where my windows 10 iso files are at. I edited the /etc/grub.d/40_custom and added a menu entry for windows, as shown below

menuentry "windows 10" {
set root=(hd0,4)
insmod part_msdos
insmod ntfs
chainloader /windows/efi/boot/bootx64.efi
}

i then did sudo update-grub and booted the efi
however, i get an error:
Windows failed to start.
Status: 0xc000000f

Also to note, I CANNOT USE USB OR DVD. I have to boot the windows iso this way,

Are you aware that you cannot boot a windows ISO this way for any purpose but installing? AFAIK there are no 'live' windows ISOs that would allow booting and running the full OS from the ISO. Windows installs then only runs from the installed OS since most of what is on the ISO is compressed files ready for installation and configuration of the installed system.

You are already running debian, why not create a VM and install windows into that VM from the ISO. Then you will be able to have both systems available as needed and no longer need to worry about the ISO, or manually creating the grub entry.

Thanks for the response, I will install the iso, get windows, and then dual boot Debian. I dont mind if my data gets deleted. How am I able to do this?

Inability to use USB or DVD is an uncommon problem. Why is it happening to you?

You need to extract or loop mount the windows iso file to a directory and then copy the files to a partition with a windows filesystem and put a proper entry in the grub.cfg menu of Debian which you already have installed. It isn't clear whether this is what you have done in your initial post as you refer to windows "iso files"?

There are numerous sites online describing the process and it is the same as creating a bootable windows installer on a usb. The link below explains it at the bottom of the page.

https://www.linuxbabe.com/desktop-li...b2-boot-loader

What i meant by iso files have the contents extracted from them when they are mounted, so like boot, efi, etc. Anyway, I have a BIOS, so i followed the instructions for installing windows 10 and having a BIOS for the grub menuentry. However, I have an error:
ntldr.mod is missing
How could i resolve this error?

In your initial post, you show a Grub menuentry which is EFI but show "insmod part_msdos". Windows won't install in EFI mode on an msdos/Legacy drive, it needs to be GPT. If the drive is GPT, change that line to insmod part_gpt.

The link below discusses possible reasons for the ntldr is missing error.

https://unix.stackexchange.com/quest...ing-from-grub2

You should have an EFI folder on the partition on which you have the extracted windows. Make sure you have the correct path and uppercase letters where needed.