This error is with package pamac-aur-git :
Preparing...
Synchronizing package databases...
Resolving dependencies...
Checking inter-conflicts...
Failed to prepare transaction:
target not found: archlinux-appstream-data
Do you want me to install and test with any other package of pamac ?
I just installed pamac-aur on a fresh install of xfce in vm. No issues. The problem sometimes can be packages and mirrors.
Edited:
Can we see the contents of /etc/pacman.conf
Can we see the output of yay -Syu pamac-aur-git? Not just the error but all of it.
It looks like it worked. Where are you seeing errors?
The error is while installing something from pamac, the ones mentioned in the post description.
OK, so we need to see the full output from the session where are you getting the error. Pamac may only show the error by default, but there should be a way to see the full output. That is what we need.
I didn’t get which output do you mean to see.
But I tried doing pamac install htop from terminal and that worked fine.
pamac install htop
Preparing...
Synchronizing package databases...
Choose optional dependencies for htop:
1: lsof: show files opened by a process
2: strace: attach to a running process
Enter a selection (default=none):
Resolving dependencies...
Checking inter-conflicts...
To install (1):
htop 3.2.1-1 extra 148.0 kB
Total download size: 148.0 kB
Total installed size: 371.1 kB
Apply transaction ? [y/N] y
Download of htop (3.2.1-1) started
Download of htop (3.2.1-1) finished
Checking keyring... [1/1]
Checking integrity... [1/1]
Loading packages files... [1/1]
Checking file conflicts... [1/1]
Installing htop (3.2.1-1)... [1/1]
Running post-transaction hooks...
Arming ConditionNeedsUpdate... [1/3]
Updating icon theme caches... [2/3]
Updating the desktop file MIME type cache... [3/3]
Transaction successfully finished.
Looks like the errors are only there when installing through gui.
Can you share a screenshot of the pamac window when you get that error?
The issue is resolved now. I think the problem lies in mirror settings.
So after I installed eos, I went to welcome settings, updated the mirrors for my region (India), and did the update and found nothing to update till now.
I just went again and changed the mirrors to worldwide and tried the pamac to install same application and it installed. Also now on doing a yay -syu I am getting a big update.
As i said… 
Yeah, I tried updating the mirrors after reading your comment
Updating and location can affect lots of things. Mirror location and how the mirrors are performing make a huge difference.
Edit: It is best to get to know what mirrors work well for your location.
True on that advice about knowing what mirrors work for you. I long time ago gave up trying to find the fastest mirror - opting for reliable mirror that hosts dozens of other projects. I can’t recall when I last ranked mirrors or used reflector, it’s been over a year.
One other concern I have with users using pamac related to the recent grub issue. During -Syu’s, if a new grub package comes down to your system, you’ll see messages in the terminal to run grub-install and grub-mkconfig with parameters for your system’s config.
I don’t know where pamac will raise those or if it even will.
Pretty sure it doesn’t have any way to handle it automatically. It might show it in the text output window, if you open it up, but the user would have to know there’s something in there to look for.