php-facedetect is available in Debian unstable, but was removed from Debian testing (which PureOS derive from) some months ago, as (vaguely, in the news section in the middle) indicated at https://tracker.debian.org/pkg/php-facedetect. The cause is that the package is weakly maintained in Debian - specifically it needs its maintainer contact address updated since the switch from Alioth to Salsa as main development environment in Debian.