initramfs-tools-core software package provides generic modular initramfs generator (core tools), 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 initramfs-tools-core
initramfs-tools-core is installed in your system.
Make ensure the initramfs-tools-core package were installed using the commands given below,
$ sudo dpkg-query -l | grep initramfs-tools-core *
You will get with initramfs-tools-core package name, version, architecture and description in a table
| Distro | Debian 9 stretch |
| Section | Utilities |
| Package | initramfs-tools-core |
| Version | 0.125 |
| Description | generic modular initramfs generator (core tools) |
| Sub Section | Utils |