It appears like you generated the mesh vertices, but did not generate any texture coordinates (aka uv map) for them. Texture coordinates are important, because they tell the renderer which part of the texture should be visible on which polygon of your 3d mesh.
So when you generate a mesh using the Mesh class, make sure that you don't just fill the
.vertices array but also the
.uv array with one
Vector2 for each
Vector3 in your vertices array.