vignette shader dons't work

here is my code I realy got stucked for a while cous I can’t make this one I’m a beginer with the shaders could you guys help me?

this is my fragmen shader:

#version 330

uniform vec4 color;

void main(void)
{
//xsize of the block 100.0 y size of the block 100.0
//vec2 position = vec2 (gl_FragCoord.x/100.0,gl_FragCoord.y/100.0)-vec2(0.5);

vec2 position = (gl_FragCoord.xy  * 0.5f);

float len = length (position);     
gl_FragColor = vec4(color.r * (1.0 - len),color.g * (1.0 - len),color.b * (1.0 - len),color.a);

}
I want to load it on each block in the game but I can’t make it