Porque você já abriu um terminal, com um comando simples
sudo apt-get update
Isso é sempre necessário se você fez algo como:
sudo add-apt-repository ppa:kivy-team/kivy
De man apt-get
update
update is used to resynchronize the package index files from their
sources. The indexes of available packages are fetched from the
location(s) specified in /etc/apt/sources.list. For example, when
using a Debian archive, this command retrieves and scans the
Packages.gz files, so that information about new and updated packages
is available. An update should always be performed before an upgrade
or dist-upgrade. Please be aware that the overall progress meter will
be incorrect as the size of the package files cannot be known in
advance.