Connecting LG27UL850 to MacBook Pro 13" 2015 at 3840x2160 60hz
A 4k resolution at 60 Hz over HDMI requires HDMI 2.0, which your MacBook Pro does not support.

Instead you'll need to use the Thunderbolt 2 port with a mini-DisplayPort to DisplayPort cable, which supports 3840x2160 at 60 Hz.