Install libqgis-app2.18.17 by entering the following commands in the terminal:
sudo apt update sudo apt install libqgis-app2.18.17
Description:
QGIS - shared app library
QGIS is a Geographic Information System (GIS) which manages, analyzes and display databases of geographic information. . This package contains the shared app library.
Homepage: https://qgis.org/
Version: 2.18.17+dfsg-1
Section: universe/libs