Welcome to fdisk (util-linux 2.33.1).
Changes will remain in memory only, until you decide to write them.
Be careful before using the write command.
Device does not contain a recognized partition table.
Created a new DOS disklabel with disk identifier 0xa82cc7ff.
Command (m for help): Partition type
p primary (0 primary, 0 extended, 4 free)
e extended (container for logical partitions)
Select (default p): Partition number (1-4, default 1): First sector (2048-1490943, default 2048): Last sector, +/-sectors or +/-size{K,M,G,T,P} (2048-1490943, default 1490943):
Created a new partition 1 of type 'Linux' and of size 128 MiB.
Command (m for help): Selected partition 1
Hex code (type L to list all codes): Changed type of partition 'Linux' to 'W95 FAT32'.
Command (m for help): Selected partition 1
The bootable flag on partition 1 is enabled now.
Command (m for help): Partition type
p primary (1 primary, 0 extended, 3 free)
e extended (container for logical partitions)
Select (default p): Partition number (2-4, default 2): First sector (264192-1490943, default 264192): Last sector, +/-sectors or +/-size{K,M,G,T,P} (264192-1490943, default 1490943):
Created a new partition 2 of type 'Linux' and of size 599 MiB.
Command (m for help): Disk Raspbian-stretch-rpix.img.tmp: 728 MiB, 763363328 bytes, 1490944 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0xa82cc7ff
Device Boot Start End Sectors Size Id Type
Raspbian-stretch-rpix.img.tmp1 * 2048 264191 262144 128M b W95 FAT32
Raspbian-stretch-rpix.img.tmp2 264192 1490943 1226752 599M 83 Linux
Command (m for help): The partition table has been altered.
Syncing disks.
Formatting boot partition on /dev/loop0...
mkfs.fat 4.1 (2017-01-24)
losetup: Raspbian-stretch-rpix.img.tmp: failed to set up loop device: Resource temporarily unavailable
make[1]: [rootfs.mak:181: Raspbian-stretch-rpix] Error 1
make: [Makefile:21: build-rootfs] Error 2
I can't speak to this, as I don't use virtual box, if virtual box uses a OPENVZ style setup then the kernel is shared and loopbacks might not be possible.
I used a VirtualBox to build the image.
Debian version: 10.8 Linux raspberry-pi 4.19.0-14-amd64 #1 SMP Debian 4.19.171-2 (2021-01-30) x86_64 GNU/Linux
Welcome to fdisk (util-linux 2.33.1). Changes will remain in memory only, until you decide to write them. Be careful before using the write command.
Device does not contain a recognized partition table. Created a new DOS disklabel with disk identifier 0xa82cc7ff.
Command (m for help): Partition type p primary (0 primary, 0 extended, 4 free) e extended (container for logical partitions) Select (default p): Partition number (1-4, default 1): First sector (2048-1490943, default 2048): Last sector, +/-sectors or +/-size{K,M,G,T,P} (2048-1490943, default 1490943): Created a new partition 1 of type 'Linux' and of size 128 MiB.
Command (m for help): Selected partition 1 Hex code (type L to list all codes): Changed type of partition 'Linux' to 'W95 FAT32'.
Command (m for help): Selected partition 1 The bootable flag on partition 1 is enabled now.
Command (m for help): Partition type p primary (1 primary, 0 extended, 3 free) e extended (container for logical partitions) Select (default p): Partition number (2-4, default 2): First sector (264192-1490943, default 264192): Last sector, +/-sectors or +/-size{K,M,G,T,P} (264192-1490943, default 1490943): Created a new partition 2 of type 'Linux' and of size 599 MiB.
Command (m for help): Disk Raspbian-stretch-rpix.img.tmp: 728 MiB, 763363328 bytes, 1490944 sectors Units: sectors of 1 * 512 = 512 bytes Sector size (logical/physical): 512 bytes / 512 bytes I/O size (minimum/optimal): 512 bytes / 512 bytes Disklabel type: dos Disk identifier: 0xa82cc7ff
Device Boot Start End Sectors Size Id Type Raspbian-stretch-rpix.img.tmp1 * 2048 264191 262144 128M b W95 FAT32 Raspbian-stretch-rpix.img.tmp2 264192 1490943 1226752 599M 83 Linux
Command (m for help): The partition table has been altered. Syncing disks.
Formatting boot partition on /dev/loop0... mkfs.fat 4.1 (2017-01-24) losetup: Raspbian-stretch-rpix.img.tmp: failed to set up loop device: Resource temporarily unavailable make[1]: [rootfs.mak:181: Raspbian-stretch-rpix] Error 1 make: [Makefile:21: build-rootfs] Error 2