This one isn’t available in the repositories anymore:
$ mhwd-kernel -l
available kernels:
* linux419
* linux510
* linux515
* linux54
* linux61
* linux65
* linux66
* linux67
* linux61-rt
* linux65-rt
* linux66-rt
So it would seem you have skipped quite a few updates. So refresh the mirror and mirror list, like @brahma said:
And then attempt a full upgrade:
pamac upgrade && pamac upgrade --aur --devel || echo -e '\033[0;91mThere was an error upgrading the system. AUR packages not upgraded.\e[0m'
This will update your system and ONLY if that was successful update any AUR packages.
a Reminder: While use of the AUR is possible, it’s neither recommended nor supported.
I recommend pamac instead of pacman, especially for newcomers as pamac was developed by Manajaro (developers) for Manjaro and just takes care of more thing than pacman. For example: