Have you tried updating the os from the Mac App Store but apple probably removed high Sierra from the App Store
read the faq
Citra is actively tested and supported on various 64-bit version of Windows ( 7 and up), Linux and macOS 10.13 High Sierra or higher. Other platforms may work, but aren’t tested by developers. In far future, mobile platforms may be targeted as well. We have no plans to support 32-bit operating system officially.
The only hard requirements for the official version of Citra is a graphics chip that supports at least OpenGL 3.3 and a 64-bit OS running on a standard PC, but you definitely want a processor with the highest possible performance per core.