Please see the image below, is there a preseed directive to skip this Debian9 installer step?
Currently I use the following config relevant to the package manager:
### Apt setup
d-i apt-setup/non-free boolean true
d-i apt-setup/contrib boolean true
d-i apt-setup/local0/source boolean true
### Package selection
tasksel tasksel/first multiselect standard
### Mirror settings
d-i mirror/country string manual
d-i mirror/http/hostname string httpredir.debian.org
d-i mirror/http/directory string /debian
d-i mirror/http/proxy string
