I cant make an stencil using texture painting

I had the same problem, but then I realized, that my normals where flipped. So to be able to draw with the stencil method, make shure that the normals of your mesh are oriented the right way (blue) or turn off backface culling in the brush settings (active tool and worspace settings > options > backface culling).

There are two common methods to use a PNG as stencil on a model, and I think you're confusing the two together.

One is to use the alpha channel as Fac to choose which color (or material) to show; in this case, no texture paint is needed, and the location of the stencil will be decided by moving the UV, eventually using the possibility of multiple UV maps, creating one to be used exclusively for the material of the stencil zone.

The Other is to use the PNG image as a stencil in texture paint mode; in this case, the PNG has to be loaded into a texture, and this texture has to be loaded in the texture slot of the brush settings, while the material is simply the texture you painted, without any need of alpha channel. Remember to turn its mapping to "stencil" while painting and use the Right Mouse Button with Shift and Ctrl modifiers to move rotate and scale your stencil.