We had previously announced that OpenGL support on Mac was deprecated in UE4 4.14 and it was removed entirely in UE4 4.15. That means from UE4 4.15 we require a Mac with a Metal compatible GPU as documented by Apple - with the following addendum:
- Thunderbolt equipped Macs running macOS High may work via an External GPU.
- 2010 or later with an upgraded GPU (Nvidia 6XX or later, AMD 7950 Mac Edition or later) may also work but are officially unsupported by Apple.
In your specific case, an Nvidia 320M, was never officially supported by UE4 on macOS as the driver for this GPU was never updated to support OpenGL 4.1 which was the previous minimum requirement for Mac UE4.
Other operating systems have different driver & OpenGL stacks that provide different levels of support for UE4.