Adding BRDF lighting to custom Terrain Shader.

So I have this modified terrain shader that has toon outlines, amongst some other changes. But how would I add BRDF to this?

I tried to add it. The error it gives me is: "Too many texture interpolators would be used for ForwardBass pass."

What am I doing wrong, I am new to working with shaders. Or does some one have an example of a Terrain shader with brdf?

Please point me in the right direction with some feedback. Your help would be greatly appreciated.

here is the code:

  Shader "Hidden/TerrainEngine/Splatmap/Lightmap-FirstPass" {
     Properties {
         _Control ("Control (RGBA)", 2D) = "red" {}
         _Splat3 ("Layer 3 (A)", 2D) = "white" {}
         _Splat2 ("Layer 2 (B)", 2D) = "white" {}
         _Splat1 ("Layer 1 (G)", 2D) = "white" {}
         _Splat0 ("Layer 0 (R)", 2D) = "white" {}
         // used in fallback on old cards
         _MainTex ("BaseMap (RGB)", 2D) = "white" {}
         
         _Detail ("Texture", 2D) = "white" {}
         _Blend ( "Blend", Range ( 0, 1 ) ) = 0.001
         
         _OutlineColor ("Outline Color", Color) = (0,0,0,1)
         _Outline ("Outline width", Range (.002, 0.03)) = .001
         _Color ("Main Color", Color) = (1,1,1,1)
         _ToonShade ("ToonShader Cubemap(RGB)", CUBE) = "" { Texgen CubeNormal }
         
         _Lookup ("BRDF Color Ramp", 2D) = "gray" {}
     }
     
     CGINCLUDE
         #include "UnityCG.cginc"
         struct appdata {
             float4 vertex : POSITION;
             float3 normal : NORMAL;
         };
         struct v2f {
             float4 pos : POSITION;
             float4 color : COLOR;
         };
         uniform float _Outline;
         uniform float4 _OutlineColor;
         v2f vert(appdata v) {
             v2f o;
             o.pos = mul(UNITY_MATRIX_MVP, v.vertex);
             float3 norm = mul ((float3x3)UNITY_MATRIX_IT_MV, v.normal);
             float2 offset = TransformViewToProjection(norm.xy);
             o.pos.xy += offset * o.pos.z * _Outline;
             o.color = _OutlineColor;
             return o;
         }
     ENDCG
 
     SubShader {
     Tags {
         "SplatCount" = "4"
         "Queue" = "Geometry-100"
         "RenderType" = "Opaque"
     }
     Pass {
         Name "OUTLINE"
         Tags { "LightMode" = "Always" }
         Cull Front
         ZWrite On
         ColorMask RGB
         Blend SrcAlpha OneMinusSrcAlpha
         CGPROGRAM
                 #pragma vertex vert
                #pragma fragment frag
               half4 frag(v2f i) :COLOR { return i.color; }
           ENDCG
     }
     
     
     CGPROGRAM
     #pragma surface surf Ramp
     
        float _Blend;
        sampler2D _Detail;
        
         sampler2D _Ramp2D;  //<======= and this
        
     struct Input {
         float2 uv_Control : TEXCOORD0;
         float2 uv_Splat0 : TEXCOORD1;
         float2 uv_Splat1 : TEXCOORD2;
         float2 uv_Splat2 : TEXCOORD3;
         float2 uv_Splat3 : TEXCOORD4;
         float4 screenPos;
     };
     //this is what i added to try brdf===============================================
     half4 LightingRamp (SurfaceOutput s, half3 lightDir,half3 viewDir, half atten){
         
             float NdotL = dot(s.Normal, -lightDir);
             float NdotE = dot(s.Normal, viewDir);
             
             //do diffuse wrap here
             float diff = (NdotL * 0.3) +0.5;
             float2 brdfUV = float2(NdotE*0.8,diff);
             float3 BRDF = tex2D(_Ramp2D, brdfUV.xy).rgb;
             
             float4 c;
             c.rgb =  BRDF; 
             //float3(diff,diff,diff);
             c.a = s.Alpha;
             return c;
         
         }
     //=======================================================================
     
     
     sampler2D _Control;
     sampler2D _Splat0,_Splat1,_Splat2,_Splat3;
     void surf (Input IN, inout SurfaceOutput o) {
     fixed4 splat_control = tex2D (_Control, IN.uv_Control);
     fixed3 col;
     col = splat_control.r * tex2D (_Splat0, IN.uv_Splat0).rgb;
     col += splat_control.g * tex2D (_Splat1, IN.uv_Splat1).rgb;
     col += splat_control.b * tex2D (_Splat2, IN.uv_Splat2).rgb;
     col += splat_control.a * tex2D (_Splat3, IN.uv_Splat3).rgb;
     //o.Albedo = col;
     
     float2 screenUV = IN.screenPos.xy / IN.screenPos.w;
                   screenUV *= float2(1,1);
     fixed4 t2 = tex2D( _Detail, screenUV)*2;
     //o.Albedo = lerp( col, t2, _Blend);
     o.Albedo = lerp( col, t2, .1);
     
     o.Alpha = 0.0;
     }
     ENDCG
     }
     SubShader {
     Tags { "RenderType"="Opaque" }
     UsePass "Toon/Basic/BASE"
     Pass {
     Name "OUTLINE"
     Tags { "LightMode" = "Always" }
     Cull Front
     ZWrite On
     ColorMask RGB
     Blend SrcAlpha OneMinusSrcAlpha
     CGPROGRAM
     // Upgrade NOTE: excluded shader from OpenGL ES 2.0 because it does not contain a surface program or both vertex and fragment programs.
     #pragma exclude_renderers gles
     #pragma vertex vert
     #pragma exclude_renderers shaderonly
     ENDCG
     SetTexture [_MainTex] { combine primary }
     }
     }
     // Fallback to Diffuse
     Fallback "Diffuse"
     }