1. Open a Main Window in pgAdmin and then open another Query Tools Window

2. In the main windows in pgAdmin,

Disconnect the "TemplateDB" database that you want to use as a template.

3. Goto the Query Tools Window

Run 2 queries as below

SELECT pg_terminate_backend(pg_stat_activity.pid) FROM pg_stat_activity WHERE pg_stat_activity.datname = 'TemplateDB' AND pid <> pg_backend_pid();

(The above SQL statement will terminate all active sessions with TemplateDB and then you can now select it as the template to create the new TargetDB database, this avoids getting the already in use error.)

CREATE DATABASE 'TargetDB' WITH TEMPLATE='TemplateDB' CONNECTION LIMIT=-1;

1. Open the Main Window in pgAdmin and then open another Query Tools Window
2. In the main windows in pgAdmin,

Disconnect the "templated" database that you want to use as a template.

3. Goto the Query Tools Window

Run 2 queries as below

SELECT pg_terminate_backend(pg_stat_activity.pid) 
    FROM pg_stat_activity 
    WHERE pg_stat_activity.datname = 'TemplateDB' AND pid <> pg_backend_pid(); 

(The above SQL statement will terminate all active sessions with TemplateDB and then you can now select it as the template to create the new TargetDB database, this avoids getting the already in use error.)

CREATE DATABASE 'TargetDB'
  WITH TEMPLATE='TemplateDB'
       CONNECTION LIMIT=-1;