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