Larian Studios Forums Divinity: Original Sin 2 Divinity - Original Sin 2 - Bugs & Technical Problems macOS currently known issues and limitations

Unfortunately, your GPU does not have Metal support, so will not be able to run the game. It may be possible to upgrade the card, or use an eGPU with Metal support.
While SSE and AVX instruction support was added for 2010/2012 CPUs which do not have AVX2 support (with the Steam and GOG releases), officially iMacs (with AMD GPUs) and Mac Pro models are only supported since late 2013.