Combining two manifold objects creates a non-manifold with issues I'm trying to combine these objects to be non-manifold for 3D printing but I'm not sure why it keeps saying the edge loop has issues.
Both objects are non-manifold and were joined to make one object.

I used the Bridge Edge loops command to join them


When I join them they become Non-manifold
How can I make this manifold?  Or how should I join the objects together so they become Non-manifold?

When I try to delete the internal face / zero face of the Bolt as recommend that just makes the M8 Bolt Non-manifold

PS:
I'm making a 3D printed adapter that has a M8 bolt on the end of it (the M8 bolt head was created using the Bolt factory Blender addon.
I've also attached the blend file.
Firstly I think you have your terminology backwards. non manifold is bad, manifold is good.
It looks to me that you are trying to attach a bolt head to a cylinder. The problem with using bridge edge loops to do this is that the cylinder has empty space on the inside while the nut does not. this means that you are asking the printer to print faces with 0 thickness, which is impossible so the slicer ignores it or bugs out.
In order to make that a manifold print I would recommend doing 2 things.
Firstly select the bottom face of your bolt and hit I to create an extra perimeter.

now that you have 2 perimeters you can bridge both the inside and outside of the cylinder.
make sure you remember to fill the hole that this creates in the bottom of the nut.

Finally go inside and delete any faces edges or vertices that you would consider to be inside the final 3d printed item.

That should give you a manifold model.