How to pair Logitech Unifying receiver Keyboard & Mouse on Ubuntu

First, install Solaar (a program to manage Logitech devices) using

sudo apt -y install solaar

Now, run solaar by entering solaar in the start menu or Alt+F2 dialog or a shell.

The Solaar window will appear, and automatically pair available devices:

You can pair additional devices or change settings by using Solaar.

See the Solaar documentation for more info.