@asker77 and everyone else: You have to enable the canonical partners repository in order to install adobe-flashplugin. You can do everything with these three commands: sudo add-apt-repository "deb http://archive.canonical.com/ubuntu `sed -n '/UBUNTU_CODENAME/s/.*=//p' /etc/os-release` partner" sudo apt update sudo apt install adobe-flashplugin Then restart Opera. The first command adds the repository. You can also do that graphically by following these instructions, but the command is easier (cred to Ruarí Ødegaard).