A meta data file (.meta) exists but its asset ‘Packages/com.unity.2d.animation/Editor/Skinning Module/Lib/x64/BoundedBiharmonicWeightsModule.dll’ can’t be found. When moving or deleting files outside of Unity, please ensure that the corresponding .meta file is moved or deleted along with it.
just do what it says, delete the meta file. its not a big deal.