I faced the same problem. This is how I resolved it.
I opened Application Stack Builder.
I chose the installation that I need to add PostGIS to.

Under Spatial Extensions, I chose the PostGIS bundle that I needed.

Once the installation was done, I opened pgAdmin, Under the database that I was working on, I right clicked on Extensions. Clicked on create -> extension

- Selected postgis in the Name dropdown. Clicked on Save. And done.
