Kernel 5.14 is EOL, so it’s pointless to try installing it. Better is to use either 5.4, 5.10 or 5.15. The former two are LTS kernels, and the latter is scheduled to become LTS when 5.16 is officially declared stable.
That is usually the result of an interrupted update process. The kernels are only installed at the end of the update process, but the old kernels are removed at the beginning of the update process. So if you interrupt the process, then you are left with an unbootable system.
What you can do is boot up from the installer medium in live mode. Then, open up a terminal and issue the following commands…
sudo su -
manjaro-chroot -a
pacman-mirrors -f 5 && pacman -Syyu
mhwd-kernel -i linux54 linux510
update-grub
sync
exit
systemctl reboot
Don’t forget to cross your fingers… 
