Okay, I get it. Vivaldi is no longer in the Mint repositories (repos). The repositories are what you are seeing when running Synaptic. For Vivaldi, you'll have to download it from
their website and save the .deb file to your Downloads folder. When the download is complete, I think you can just double-click the .deb file to open it up and install it (using gdebi program, which I think is now included in Mint).
We'll have to save the Synaptic lesson for another program sometime. Do you like Solitaire?