Going the wrong way.... (edited /boot/linuz(?)-5.4, probably /boot/grub/ (?) etc..)
Anyway...
the easy way:
manjaro settings manager > uninstall kernel 5.4 rt > and installed again
Everything should work just fine.
Note: if you have more kernels ex 5.5 you must select (when booting on grub) the kernel to boot, with
[Esc] > Options > kernel of your choice, like kernel 5.4rt
If something with manjaro-settings-manager didn't go well (don't think so)
you can run
sudo mkinitcpio -p
(for all installed kernels) or
sudo mkinitcpio -p linux54rt
(only for selected kernel)
and
sudo update-grub and reboot
If you need to edit something for grub you do it by editing /etc/default/grub (as root) and not direct to /boot like you did, and then again
sudo update-grub and reboot
Anyway if you still want to do it you way (?) then your initramfs would be on /boot with name initramfs-5.4rt-x86_64.img
If I understand correctly you have more linux distros, so be careful when update grub in either distro that can see the other distro (meaning that if you are on manjaro and your debian partition is encrypted you must first mount it).
If you still have a problem post more info like result for commands
fdisk -l
lsblk
cat /etc/default/grub
sudo cat /boot/grub/grub.cfg
ls -l /boot
also post the current state, can you boot to linux (manjaro or debian) or usb-stick or installation-media or you are stuck at grub?
Welcome to Manjaro forum!! (this should be the first
, but i read your post, just before going to bed...)