VFX - switch texture

Hello everyone. Please help me to solve this problem. I have a solid VFX under Unity in order to display a lot of particles in the game. You can see the render taking a look at the main page project above. However, trying to change particle Texture according to a switch integer value, it does not work and I have this error message :

Shader error in ‘Hidden/VFX/Points/System/Output Particle|Unlit Quad’: undeclared identifier ‘mainTexture’ at Assets/VFX/Points.vfx(2621) (on d3d11)

What is the problem. This way I set texture no? Thanks for your help ++