elpa-projectile software package provides project interaction library for Emacs, 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 elpa-projectile
elpa-projectile is installed in your system.
Make ensure the elpa-projectile package were installed using the commands given below,
$ sudo dpkg-query -l | grep elpa-projectile *
You will get with elpa-projectile package name, version, architecture and description in a table
| Distro | Debian 9 stretch |
| Section | Lisp |
| Package | elpa-projectile |
| Version | 0.14.0-1 |
| Description | project interaction library for Emacs |
| Sub Section | Lisp |