Error Meshing for Mirror
 

Hello,

I'm doing a meshing for a cubic, with 1/2 pellet and 1/4 pellet on the sides as shown in the first image. I use an unstructured mesh (tetra and prism) and mirroring the sides (Edit Mesh>Transform Mesh>Mirror Mesh). I didn't get anything looks weird from the meshing form but when I run it, I got an error message as shown in the second image. I don't know which part is wrong, whether it's from the meshing or the mirror steps. Can anyone suggest me a way to find out which part is incorrect and fix it from an error message? Any suggestions will be helpful, thanks in advance!

You could try using a symmetry plane if you want to go that route.