Install akonadi-backend-sqlite by entering the following commands in the terminal:
sudo apt update sudo apt install akonadi-backend-sqlite
SQLite 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 SQLite as underlying data storage engine. Since SQLite is an embedded database engine, a separate SQL server daemon is not necessary. . In addition, the package contains an improved QSql driver for SQLite named "QSQLite3". It can be used by any application that needs to access SQLite databases via standard Qt QSql framework.