I want to make part of the mesh transparent. For this I used 2 principled bsdf nodes. They are linked to a mix shader. The transparent bsdf to the 2nd slot, the leather to the 1st slot. I've made a mask/ texture image with black background and I painted white where I wanted the mesh to be transparent, whose color slot is linked to the mix shader fac slot. The normal slot from the normal map of the leather material is connected to both principled bsdf normal slots. Any guess what I did wrong for this ? The top principled bsdf is for the transparent part, the bottom is the leather.
It doesn't work because your "transparent" Principal BSDF is not transparent because the Alpha slot value is 1.0. It should be 0.0. But you don't need to mix 2 Principled BSDF shaders for transparency.
You just need one Principled BSDF. Then plug the mask (Image Texture node) into the Alpha slot of the Principled BSDF node. That's all :-)
Of course, you can do it as you did. But you better would take a Transparent BSDF (instead of another Principled BSDF) and mix it with a Principled BSDF. The mask goes into the Fac slot of the Mix Shader as you did. Such setups are often used for glass and plastic shaders.
Transparent shader nodes don't need a normal map. (At least I've never this.)