update-initramfs -u -k `uname -r`
You could change -k to a specific kernel name if for some reason the current is not running (eg. if you are chrooted or in recovery mode).
If you want to update all kernels then use "-k all"
Sometimes old entries persist in /var/lib/initramfs-tools so the solution is to delete the invalid entry. This is important is often DKMS or other updates when installing packages may want to rebuild initramfs for all known kernels but this will all fail if it tries to build an old non-existent kernel it finds in /var/lib/initramfs-tools
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