Material doesn't have a color property '_Color'

 Shader "kaiyum/cg basic shader"
 {
     Properties 
     {
       _Color ("Color", Color) = (1, 1, 1, 0.5) 
     }
 SubShader
 {
 Tags { "Queue" = "Transparent" }
 Pass 
  { 
    ZWrite Off
    Blend SrcAlpha OneMinusSrcAlpha
    CGPROGRAM
    #pragma vertex vert
    #pragma fragment frag
    #include "UnityCG.cginc"
    
    uniform float4 _Color;
    struct vin
    {
      float4 vertexpos : POSITION;
      float3 normal : NORMAL;
    };
    struct vout
    {
      float4 vertexpos : SV_POSITION;
      float3 normal : TEXCOORD0;
      float3 viewdir : TEXCOORD1;
    };
    
    vout vert(vin i)
    {
      vout o;
      o.vertexpos=mul(UNITY_MATRIX_MVP,i.vertexpos);
      o.normal=normalize(mul(float4(i.normal,0.0),_World2Object));
      o.viewdir=normalize(_WorldSpaceCameraPos-float3(mul(_Object2World,i.vertexpos)));
      return o;
    
    }
    
    float4 frag(vout o):color
    {
      float3 nd=normalize(o.normal);
      float3 vd=normalize(o.viewdir);
      float curThres=abs(dot(nd,vd));
      if(curThres > 0.2){discard;}
      else
      {
      float newOpacity = min(1.0,_Color.a/abs(dot(vd,nd)));
      return float4(float3(_Color),newOpacity);
      
      }
    }
    ENDCG
   }
 }
 
 }

I have examined it several times. I do not know why unity does not compile my shader!! Can anybody point me here?