PrecomputedAOMask not working in 4.13 and 4.14
I had no issues getting PrecomputedAOMask working in 4.13 or 4.14. Could you make sure that you have enabled both Use Ambient Occlusion and Generate Ambient Occlusion Material Mask under World Settings → Lightmass settings, and then build lighting. For further information you can check our documentation.
You may need to adjust the Light mass settings a bit to get the AO results you want.