To create/recreate/update the initramfs file means to update the initrd.img-* ramdisk files in /boot.

Here's a listing of my /boot. Note the size/date of the initrd.img-* files. One or more of these need updating in your case.

~$ ls -al /boot

total 162745

drwxr-xr-x  4 root root     4096 Jul  5 05:19 .

drwxr-xr-x 23 root root     4096 Jun 19 12:40 ..

-rw-r--r--  1 root root   252994 May  7 05:38 config-5.11.0-18-generic

-rw-r--r--  1 root root   253022 Jun 16 15:38 config-5.11.0-22-generic

drwxr-xr-x  3 root root     1024 Dec 31  1969 efi

drwxr-xr-x  5 root root     4096 Jun 30 12:09 grub

lrwxrwxrwx  1 root root       28 Jun 25 12:42 initrd.img -> initrd.img-5.11.0-22-generic

-rw-r--r--  1 root root 62066094 Jul  5 05:19 initrd.img-5.11.0-18-generic

-rw-r--r--  1 root root 62067859 Jul  5 05:18 initrd.img-5.11.0-22-generic

lrwxrwxrwx  1 root root       28 Jun 25 12:42 initrd.img.old -> initrd.img-5.11.0-18-generic

-rw-r--r--  1 root root   182704 Aug 18  2020 memtest86+.bin

-rw-r--r--  1 root root   184380 Aug 18  2020 memtest86+.elf

-rw-r--r--  1 root root   184884 Aug 18  2020 memtest86+_multiboot.bin

-rw-------  1 root root  5968565 May  7 05:38 System.map-5.11.0-18-generic

-rw-------  1 root root  5969489 Jun 16 15:38 System.map-5.11.0-22-generic

lrwxrwxrwx  1 root root       25 Jun 25 12:42 vmlinuz -> vmlinuz-5.11.0-22-generic

-rw-------  1 root root 14737344 May  7 06:26 vmlinuz-5.11.0-18-generic

-rw-------  1 root root 14741312 Jun 16 15:55 vmlinuz-5.11.0-22-generic

lrwxrwxrwx  1 root root       25 Jun 25 12:42 vmlinuz.old -> vmlinuz-5.11.0-18-generic

Note: I prefer to create a totally fresh version by using the -c option, instead of just updating the existing file by using the -u option.

The proper command would be:

sudo update-initramfs -c -k $(uname -r)

This will create a fresh initrd.img-* file for your currently booted version of Ubuntu.

However, if you can't boot to the current version of Ubuntu, you may have to modify this command, and by booting to an older version of Ubuntu, you can do it this way:

sudo update-initramfs -c -k 5.11.0-22-generic where the 5.11.0-22-generic part should be replaced with the version of the desired boot kernel.

To regenerate all of the initrd.img-* files (not recommended), use:

sudo update-initramfs -c -k all

To get more detailed information, type:

man update-initramfs