This error occurs when the mesh you are working with is not manifold and you try to do further operations with it. A manifold is a topological space that is locally Euclidean. A mesh not being manifold will cause further errors down the road or completely block processes.
A mesh can be non-manifold due to many reasons. Some of the most common occurrences are:
- An edge connected to more or less than two faces
- Separate regions connected through a single vertex
- Loose geometry (discontinuous)
- Plane normals pointing in opposite directions for adjacent faces
To check if your mesh is manifold or not, click on the info (?) panel and go to Properties. There will be a property called Manifold. If it says False, then it is not manifold and if it says True, then it is.
If it says False, you will need to refine your mesh further. Try using a smaller feature size or changing the adaptivity.
For further meshing tips, check out the articles in the Meshing Guide: Meshing Guide