How to install flash player for Netrunner Core?

Flash content not playing in Vivaldi browser. I use this browser for many years, i’ve tried to follow the instructions to enable flash content on the browser but nothing seems to work.
When i did ```
sudo add-apt-repository “deb lsb_release -cs partner”
sudo apt update
sudo apt install adobe-flashplugin

The terminal indicated ““cannot be found””

Flash Player is deprecated and highly not recommended to use nowadays.
For Vivaldi you need to manually install the ppapi flashplayer.
See also the other section here

What Leszek said, plus Netrunner core is based on Debian stable, not Ubuntu.
Here are the Debian methods of installation: