I didn’t mean you can add interior faces. What I meant is that you can add those faces I suggested you to delete back, after you apply the array. Interior faces are a type of mesh error (making the mesh non-manifold) and your mesh still has problem with them:
- I moved the array to be on top of the modifier stack. The only thing this does is that one can apply the array modifier before mirror.
- I enabled merge in the array modifier and changed the merge distance to a very small number. This connects the parts in the array.
Now that the array is visible in edit mode, you see the resulting problem (strange lines) and also what is causing it. Because this is an array, front faces get repeated. The topology is fine in the first piece but after that they cause a problem.
Selected faces make the interior faces in the array - They are connected like this
which you should always avoid.
The solution is to remove those front faces while they still are in the array.