The AUR PKGBUILDs are done for Arch Linux, NOT Manjaro. Some packages are in other branches like testing or unstable. Either edit the PKGBUILDs as needed or switch branches. Simple. For viber, simple revert this change: https://aur.archlinux.org/cgit/aur.git/commit/?h=viber&id=8a9856701d4386ec1c3193d364ea962bc069f499. You can also do like this:
mkdir -p viber && cd viber
wget https://aur.archlinux.org/cgit/aur.git/plain/PKGBUILD?h=viber&id=b175a2bd8159654ea23a17d67ad55a753b2d3b28 -O PKGBUILD
makepkg -si