Normally this procedure you have pasted should help recover. However, you didn’t specify whether or not you completed those instructions. Another procedure is noted below, if that fails.
It seems that your mirrors need to be refreshed before performing an update. From a chroot environment, try:
pamac update --no-aur --force-refresh
Note this is basically the pamac variation of the pacman command already given, but without the grub portion.
If the update completes, the problem is likely resolved, but if you also need to replace Grub, there is a new program for that install-grub (which may or may not already be installed):
pacman -S install-grub
install-grub
It’s possible that you might need to reset the default bootloader in your BIOS – presuming UEFI – enter the BIOS UEFI configuration and set UEFI OS on the Manjaro disk as the default.
Welcome to the Manjaro community
As a new or infrequent forum user, please take some time to familiarise yourself with Forum requirements, and the many ways to use the forum to your benefit.
To that end, links are provided (below) - Please use them.
Be prepared to provide outputs from various commands when asked. It’s equally important to provide as much actionable information as possible in your first post, rather than simply indicating there is a problem.
Waiting for someone to blindly ask questions is also counter-productive – typically, nobody has a 
at their disposal – Instead, please help others to ask informed questions, based on information you provide.
Update Announcements
The Update Announcements contain important information and a Known Issues and Solutions section that should generally be checked before posting a request for support.
System Information
While information from *-fetch type apps might be fine for someone wishing to buy your computer, for Support purposes it’s better to ask your system directly; 
Output of the inxi command (with appropriate parameters, and formatted according to forum guidelines) will generate information useful for those wishing to help:
inxi --filter --verbosity=8
or the short form (preferred):
inxi -zv8
- inxi man pages (Arch Manual)