Currently r8Unorm_srgb is required by virtual texture lightmap, and the Metal RHI simply forbids r8Unorm_srgb on Mac platform. Since the latest MacOS 11.0+ have the support of r8Unorm_srgb, we should make r8Unorm_srgb available in MacOS 11.0+ and have a better warning instead of crash in previous OS.
enable virtual texture lightmap and build light in Mac editor.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-109932 in the post.