$ sudo apt-get update
$ sudo apt-get install qgis-plugin-grass 

qgis-plugin-grass is installed in your system.

Make ensure the qgis-plugin-grass package were installed using the commands given below,

$ sudo dpkg-query -l | grep qgis-plugin-grass *

You will get with qgis-plugin-grass package name, version, architecture and description in a table

qgis-plugin-grass package basic information: