HI!

First of all You have to install Postgis as mentioned in geospatial module description

so run the command:sudo apt-get install postgis
But postgis depends on some other libraries like:

• Shapely
• geojson

so you may encounter some problems during the installation 

so I recommend u this tutorial on how to install PostGIS and choose the right version you might need:

https://computingforgeeks.com/how-to-install-postgis-on-ubuntu-debian/

after the installation you are now able to create the two extensions needed to initiate this module on your database manually

these are some easy steps to  follow

1- connect as user postgres :sudo su postgres 

2-Switch to psql  by running:psql

3-Connect to your database:\c database_name ;

4-Run :

       CREATE EXTENSION postgis;

       CREATE EXTENSION postgis_topology;

   hope this helps someone,