How do I solve non manifold errors with this relatively simple shape? I'm pretty new to Blender, and I have a model that I am having problems with. I have some non-manifold geometry but I don't know what's wrong. I highlight the edges but I don't see anything wrong with the edges the pointed out. Here is a screenshot:

I've tried deleting the edges, because I thought they might be duplicate, but they're not. I tried deleting the surrounding faces and regenerating the faces manually, but that doesn't help. I created this model with a face I subdivided, then extruded the nine faces. Then I cleaned it up and added a bunch of vertices and faces. I thought maybe that construction technique was bad, so I tried creating this same thing by starting with two cubes and repeatedly boolean subtracting the &quot;carving&quot; cube away from the main form. But after four or so cuts I started getting non manifold errors with this technique as well. Any ideas? If you tell me how to export just this object, I can do that and share it if that would help. Thanks in advance!
Edit:
Okay, here is the file if anyone wants to look:
https://www.dropbox.com/s/2g00zfgs5xyzdhd/non_manifold.blend?dl=0
Inspecting the mesh, I couldn't find the problems you guys mentioned. Maybe I just don't know Blender well enough to see them.
As to Make Manifold, I never saw that button there, LOL. It actually works, which is great. Thanks!
I will still be playing around with the mesh as a learning experience however. I want to figure out what is wrong with it.
I would stick with the extrusion method, personally, but you do have to be aware of doubled edges or faces, depending on how you extruded it.  I would look at it in Wireframe mode to see if you have anything like this:

The red arrow is showing a doubled-up face inside the object, so you'd want to delete any of those.
The blue arrow is showing an edge that is ACTUALLY a face (with zero height).
Have you tried simply doing a Merge By Distance to clean up any vertices that are doubled up, or just realllly close to each other?
Also, silly question, but have you tried clicking that &quot;Make Manifold&quot; button there on the right side?
If nothing helps, you could upload the blend file.  If you don't want to upload it because there are other objects in the scene, you could save a copy and delete everything else.