Using manjaro-chroot -a from a Live ISO/USB has been suggested; please find following more detail for clarification and reference, if needed:
Once you have successfully entered a chroot environment, you will be the Super User (root) and able to run commands from within your actual Manjaro system.
You are then able, for example, to perform a system update if it had previously not completed:
pacman -Syu
Notice that sudo is not used; because you would already be the Super User.
These tutorials also employ entering a chroot environment in order to solve issues; in case they may be useful: