I found this question on stack overflow and answered it – then the pedantic site moderators deleted it. So here is the answer again.
On a clean install of postgres and postgis you will see this error because postgis doesn’t have the dependency to pull in the shared library. When you go into psql the command
my_database=# CREATE EXTENSION postgis;
ERROR: could not access file “$libdir/postgis-2.1”: No such file or directory
found I also needed
sudo apt-get install postgresql-9.4-postgis-2.1
I found it by looking at
apt-cache search postgres | grep gis