Texture Painting Nightmares. Any ideas on what is going on?

Okay, so I’m using Blender 2.9. Up until now, I’ve mostly used shader nodes to color my models because I was afraid of UV Mapping and Texture painting. Now I’m learning UV Mapping and Texture painting, but in Blender is has been SUPER frustrating. For example, I have a character. I selected his shoe, put it into Edit Mode, and did the Smart UV Project. Once that was done, I created a texture node in the Shader tab and created a base color texture. I duplicated that and added a Mixed RGB with a clear alpha so that I can paint on top of the base texture. I can paint directly on the UV map, but it will not let me paint on the 3D Mesh.
As a test, I created a new file, with just a Rounded Quadrasphere. I UV unwrapped it the same way, set up my nodes the same way, and I can paint directly on the 3D mesh and the UV Map both.
It’s driving me insane because I’ve put a lot time into this model, and I should be able to paint directly on the mesh. And I have made sure that I have the right texture slot selected under the paintbrush tool too. Are there any addons that make this process cut and dry that might work?

maybe pack the images and share the shoe? https://pasteall.org/blend/

Okay. I will do that here in a little bit.

Here is the shoe. I even copied it into a new blender file and tried it. I cannot get it to work.
As for the way I’m attempting to paint, I’m using this tutorial after I’ve UV Unwrapped the shoe:

Layered Texture Painting in Blender

The Shoe that I can’t Texture paint…

You need to recalculate the normals, they were flipped, then unwrap with Smart UV Project for example, give it a material with an Image Texture, and once you are in Texture Paint mode, in the N panel > Texture Slots, select the image texture you’ve just created.

Awesome!! Thank you so much for the help. I didn’t even think about the normals being flipped. I’m pretty new at this still. I’ve only been teaching myself Blender for about 4 months now.
I appreciate you. Thank you.