Install akonadi-server by entering the following commands in the terminal:
sudo apt update sudo apt install akonadi-server
Akonadi PIM storage service
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 contains the Akonadi PIM storage server and associated programs.