Solution:

Just enter the following three lines.

sudo cp /etc/apt/sources.list /etc/apt/sources.list~
sudo sed -Ei 's/^# deb-src /deb-src /' /etc/apt/sources.list
sudo apt-get update

Users who are not logged in can copy the code below.

sudo cp /etc/apt/sources.list /etc/apt/sources.list~
sudo sed -Ei ‘s/^# deb-src /deb-src /’ /etc/apt/sources.list
sudo apt-get update

Reference:
https://askubuntu.com/questions/496549/error-you-must-put-some-source-uris-in-your-sources-list