Why would you install Pamac from AUR on Manjaro? Anyway even if that would make any bit of sense, AUR is UNSUPPORTED. Install Pamac from repositories. If you have an AUR package question, comment on the AUR page and ask the maintainer there.
What I mean is that Pamac has been rebuild for the new libalpm - so technically - on Manjaro Pamac works - we still have to be aware that minor issues may surface due to the extended database format libalpm uses.
[master ?:1] $ pamac info libpamac
Name : libpamac
Version : 11.6.4+2+g1421283-4
Description : Library for Pamac package manager based on libalpm
URL : https://github.com/manjaro/libpamac
Licenses : GPL-3.0-or-later
Repository : extra
Installed Size : 3,9 MB
Groups : --
Depends On : dbus-glib git glib2 json-glib libalpm.so=15 libsoup3 pacman-mirrors polkit appstream
Optional Dependencies : --
Required By : manjaro-application-utility pamac-cli pamac-gtk3
Optional For : --
Provides : libpamac.so=11 pamac-common libpamac-appstream.so=11 libpamac-appstream-plugin
libpamac-aur.so=11 libpamac-aur-plugin
Replaces : pamac-common
Conflicts With : libpamac-aur-plugin libpamac-appstream-plugin
Packager : Mark Wagie <mark@manjaro.org>
Build Date : søn 15 sep 2024 21:51:54 CEST
Install Date : man 16 sep 2024 08:24:21 CEST
Install Reason : Installed as a dependency for another package
Validated By : Signature
Backup files : /etc/pamac.conf