Problem with Vivaldi ppa

As a workaround, you can use --allow-unauthenticated with apt. Run the command:

sudo apt update --allow-unauthenticated

If you do not want to use --allow-unauthenticated flag every time you run sudo apt update here is a solution.

1. Press Ctrl+Alt+T to open a terminal.

2. Type the command sudo nano /etc/apt/sources.list . In this file, search for vivaldi repository.

3. Now, replace the line with:

   deb [trusted=yes] https://repo.vivaldi.com/archive/deb/ stable main
   

4. Now, press Ctrl+X followed by y and then Enter to save the files.

5. Finally, update the package list:

   sudo apt update