How to install the additional module pg_trgm
pg_trgm is an extension, so:

CREATE EXTENSION pg_trgm;


If you get the following error


  ERROR:  could not open extension control file ".../extension/pg_trgm.control":
  No such file or directory"


then you need to install the module for your operating system


Ubuntu/Debian:

sudo apt install postgresql-contrib

Redhat/Centos

sudo dnf install postgresql10-contrib

Fedora

sudo dnf install postgresql-contrib