If your computer does not meet the minimum requirements for Catalina or Mojave, you will not be able to install a supported version of macOS. If you are running an older OS, you can look at the hardware requirements for the currently supported versions of macOS to see if your computer is capable of running them:
If you are running macOS 10.11 or newer, you should be able to upgrade to at least macOS 10.15 Catalina. All UVM-owned computers running macOS 10.13 High Sierra or older must be updated or replaced as soon as possible to comply with the University Information Security Policy.