debian-installer-8-netboot-arm software package provides Debian-installer network boot images for arm64, you can install in your Debian 9 (Stretch) by running the commands given below on the terminal,
$ sudo apt-get update $ sudo apt-get install debian-installer-8-netboot-arm
debian-installer-8-netboot-arm is installed in your system.
Make ensure the debian-installer-8-netboot-arm package were installed using the commands given below,
$ sudo dpkg-query -l | grep debian-installer-8-netboot-arm*
You will get with debian-installer-8-netboot-arm package name, version, architecture and description in a table
| Distro | Debian 9 stretch |
| Section | Miscellaneous |
| Package | debian-installer-8-netboot-arm |
| Version | 20150422+deb8u4.b1 |
| Description | Debian-installer network boot images for arm64 |
| Sub Section | Miscellaneous |