Just had this problem when starting my workstation this morning.

May or may not be related to bug 1184779, which I encountered simultaneously.

> [  105.589905] apollon systemd[1]: Mounting /var/spool...
> [  105.610706] apollon systemd[1]: Mounting /var/tmp...
> [  105.612009] apollon systemd[1]: Starting File System Check on /dev/disk/by-label/git...
> ...
> [  105.626645] apollon mount[1975]: mount: /var/tmp: mount(2) system call failed: Cannot allocate memory.
> [  105.636377] apollon systemd[1]: Finished Load/Save Screen Backlight Brightness of leds:dell::kbd_backlight.
> [  105.638474] apollon systemd[1]: Mounted /.snapshots.
> [  105.640099] apollon systemd[1]: Mounted /boot/efi.
> ...
> [  105.666824] apollon systemd[1]: Failed to mount /var/tmp.
> [  105.668645] apollon systemd[1]: Dependency failed for Local File Systems.
> [  105.668705] apollon systemd[1]: local-fs.target: Job local-fs.target/start failed with result 'dependency'.

After entering emergency mode, I simply typed "mount /var/tmp", and had no
issue. Bug 1184779 persisted though (see there).

/var/tmp is just a btrfs subvolume.

# btrfs subvol list /
ID 257 gen 930369 top level 5 path @
ID 258 gen 1432317 top level 257 path @/.snapshots
ID 260 gen 1427487 top level 257 path @/boot/grub2/x86_64-efi
ID 261 gen 1432361 top level 257 path @/opt
ID 262 gen 1427487 top level 257 path @/srv
ID 263 gen 18 top level 257 path @/tmp
ID 264 gen 1432300 top level 257 path @/usr/local
ID 265 gen 1432339 top level 257 path @/var/cache
ID 266 gen 1427487 top level 257 path @/var/crash
ID 267 gen 1427487 top level 257 path @/var/lib/mailman
ID 268 gen 1427487 top level 257 path @/var/lib/mariadb
ID 269 gen 1427487 top level 257 path @/var/lib/mysql
ID 270 gen 1427487 top level 257 path @/var/lib/named
ID 271 gen 1427487 top level 257 path @/var/lib/pgsql
ID 272 gen 1432392 top level 257 path @/var/log
ID 273 gen 1427487 top level 257 path @/var/opt
ID 274 gen 1432391 top level 257 path @/var/spool
ID 275 gen 1432366 top level 257 path @/var/tmp
ID 276 gen 1427487 top level 257 path @/var/lib/machines
ID 1650 gen 1432304 top level 257 path @/var/lib/systemd/coredump
ID 1893 gen 1432387 top level 258 path @/.snapshots/1113/snapshot
ID 2158 gen 1418255 top level 258 path @/.snapshots/1271/snapshot
ID 2159 gen 1424575 top level 258 path @/.snapshots/1272/snapshot
ID 2163 gen 1427385 top level 258 path @/.snapshots/1273/snapshot
ID 2164 gen 1429845 top level 258 path @/.snapshots/1274/snapshot
ID 2165 gen 1429883 top level 258 path @/.snapshots/1275/snapshot
ID 2166 gen 1429959 top level 258 path @/.snapshots/1276/snapshot
ID 2167 gen 1429963 top level 258 path @/.snapshots/1277/snapshot

It's listed in fstab like any other subvols.

> UUID=ad544c37-37f9-44a4-9798-d29d3cb5db44 /var/tmp btrfs subvol=@/var/tmp 0 0
> UUID=ad544c37-37f9-44a4-9798-d29d3cb5db44 /.snapshots btrfs subvol=@/.snapshots 0 0
> UUID=ad544c37-37f9-44a4-9798-d29d3cb5db44 /var/lib/machines btrfs subvol=@/var/lib/machines 0 0

Full boot log is attached. 

systemd-246.13-1.1.x86_64
kernel-default-5.11.11-1.2.x86_64
libmount1-2.36.2-1.3.x86_64
util-linux-2.36.2-1.3.x86_64

The system has 8BiB RAM. It seems highly unlikely that the system went out of
memory this early during boot. After boot and graphical user login, mem usage
is like this:

# free
               total        used        free      shared  buff/cache  
available
Mem:         7897336     3457336      642880      767688     3797120    
3305920
Swap:        8383484       23040     8360444