If your are using NVIDIA graphics card and trying to update macOS High Sierra to macOS Mojave the installer might show an error message that Metal is not supported despite installed graphics card has Metal support.

In order to fix it you need to install NVIDIA Web Drivers - please see Installing NVIDIA Web Driver / CUDA driver under macOS.

IMPORTANT: Make sure to run Mojave installer directly from High Sierra, not from bootable USB stick. Otherwise it won't detect Metal properly.