Setting correct permissions for /var directory recursively - #2 by megavolt
This is a very recent pertinent post from user @megavolt
perhaps in the course of your attempts … you managed to set the wrong permissions on that directory?
anyway - here is the description of the cause for the error - and the link gives the tools and shows how you can fix it
You where concerned about something regarding the grub or kernel installation - I tried to point to the file where the relevant messages will still be.
If you still have the same terminal open, you could just scroll up …
edit:
or, you could simply just reinstall that kernel and review the output again …
(probably via mhwd-kernel - but don’t ask me for specifics
I only run Manjaro in a VM and am not very familiar with the Manjaro tools)