The PKGBUILD file contains the line
depends=('php<7.2' 'php-gd<7.2')
where pacman is unable to find the packages titled
php<7.2
or
php-gd<7.2
as scene here:
$this->bbcode_second_pass_quote('', '$') sudo pacman -S nextcloud
resolving dependencies...
warning: cannot resolve "php<7.2", a dependency of "nextcloud"
warning: cannot resolve "php-gd<7.2", a dependency of "nextcloud"
:: The following package cannot be upgraded due to unresolvable dependencies:
nextcloud