Can't add paint texture in substance painter on my mesh model?

I exported my mesh model from Zbrush as a high-poly .fbx file. Now I imported the .fbx file into my substance painter and can't paint any textures on it. 

How do I fix that?

Also, I already UV mapped in Maya and then Zbrush. Does that affect my texturing process as a Substance painter?

I will provide the .ssp file. Thank you.