geogebra-classic not found

In accordance with the instructions given by https://wiki.geogebra.org/en/Reference:GeoGebra_Installation, I tried installing geogebra by first using sudo apt-add-repository -u 'deb http://www.geogebra.net/linux/ stable main', and then sudo apt install geogebra-classic, but when I run the last command, I get the error message "Unable to locate package geogebra-classic". Any tips?

You have add their GPG key by

wget https://static.geogebra.org/linux/[email protected] -O - | sudo apt-key add

and then rerun update and install package:

sudo apt-get update
sudo apt-get install geogebra-classic

First, add the GPG key, then the repository, and finally install by these commands:

$ wget https://static.geogebra.org/linux/[email protected] -O - | sudo apt-key add
$ sudo apt-add-repository -u 'deb http://www.geogebra.net/linux/ stable main' 
$ sudo apt install geogebra-classic 

Thus the latest Geogebra will be installed.