Open the mesh in the mesh editor.
- In details - under LOD 0 / Build Settings (expand) and check the box for Generate Lightmap UVS.
- Scroll down a bit and click Apply Changes button.
- In details under General Settings set your Light Map Resolution to desired (64)
- In General Settings expand Advanced and set Light Map Coordinate Index to 1.
- Save the Mesh, Build Lighting only and check warnings.