Problem scenario
You try to install VirtualBox on a Linux server via the "Software" section of your GUI. It fails with an error message like this: "Unable to install VirtualBox as download failed … not (yet) available (404 Not Found)"
Solution
Run a command like one of the following depending on the type of Linux you have.
If you have a Debian distribution of Linux (such as Ubuntu or Mint): sudo apt -y update
If you have a Red Hat distribution of Linux (such as CentOS, RHEL or Fedora): sudo yum -y update