Install akonadi-backend-postgresql by entering the following commands in the terminal:
sudo apt update sudo apt install akonadi-backend-postgresql
PostgreSQL storage backend for Akonadi
Akonadi is an extensible cross-desktop Personal Information Management (PIM) storage service. It provides a common framework for applications to store and access mail, calendars, addressbooks, and other PIM data. . This package installs everything what's needed for Akonadi to work with PostgreSQL as underlying data storage engine. By default, a local PostgreSQL server instance will be started for each user. Alternatively, connection to an external PostgreSQL database is supported as well.