Invalid texels means there are exposed backfaces somewhere, visible to those texels. You can find the backfaces using the “Highlight Back-Facing Geometry” toggle in your screenshot. Might be flipped normals or something.
If you don’t want to fix your scene geometry and just want to ignore the invalid texels, you can set your materials to Double Sided, or you can change your backface tolerance.
See this for more Lightmapping Troubleshooting Guide - #16 by kristijonas_unity