I have a make file :
buildimg:
dd if=/dev/zero of=$(BUILDDIR)/$(OSNAME).img bs=512 count=93750
mformat -i $(BUILDDIR)/$(OSNAME).img -f 1440 ::
mmd -i $(BUILDDIR)/$(OSNAME).img ::/EFI
mmd -i $(BUILDDIR)/$(OSNAME).img ::/EFI/BOOT
mcopy -i $(BUILDDIR)/$(OSNAME).img $(BOOTEFI) ::/EFI/BOOT
mcopy -i $(BUILDDIR)/$(OSNAME).img startup.nsh ::
mcopy -i $(BUILDDIR)/$(OSNAME).img $(BUILDDIR)/kernel.elf :

but get e error running it.

[sudo] password for nvthielen:
dd if=/dev/zero of=bin/CustomOS.img bs=512 count=93750
93750+0 records in
93750+0 records out
48000000 bytes (48 MB, 46 MiB) copied, 0.661928 s, 72.5 MB/s
mformat -i bin/CustomOS.img -f 1440 ::
mformat: mformat.c:367: check_fs_params_and_set_fat: Assertion `clusters_fit_into_fat(Fs)' failed.
make: *** [Makefile:42: buildimg] Aborted (core dumped)

any help is welcome tnx

mformat -i $(BUILDDIR)/$(OSNAME).img ::

whit out the -f flag.

The commands work for me on a debian 11 system but you are creating a 48MB image file and trying to format it with a 1.44MB filesystem. A FAT12 filesystem is limited to <=32MB. Without the -f it uses a FAT16 which limits at 2GB