Terrain Transparency - Too many text interpolators

Your shaders model target does not support that many texture interpolators. You can specify a higher shader model target using #pragma target <shader model>, for example #pragma target 4.0, or you could try to simplify your shader so it fits in target 2.0 or 3.0.

• Manual: Shader Targets

By default, Unity compiles shaders into roughly shader model 2.0 equivalent.

#pragma target 4.0 - compile to DX10 shader model 4.0.
This target is currently only supported by DirectX 11 and XboxOne/PS4 platforms.

Note that all OpenGL-like platforms (including mobile) are treated as “capable of shader model 3.0”. WP8/WinRT platforms (DX11 feature level 9.x) are treated as only capable of shader model 2.0.

 Shader "Nature/Terrain/Diffuse"
 {
     Properties
     {
         [HideInInspector] _Control("Control (RGBA)", 2D) = "red" {}
         [HideInInspector] _Splat3("Layer 3 (A)", 2D) = "white" {}
         [HideInInspector] _Splat2("Layer 2 (B)", 2D) = "white" {}
         [HideInInspector] _Splat1("Layer 1 (G)", 2D) = "white" {}
         [HideInInspector] _Splat0("Layer 0 (R)", 2D) = "white" {}
         //Used in fallback on old cards & base map
         [HideInInspector] _MainTex("BaseMap (RGB)", 2D) = "white" {}
         [HideInInspector] _Color("Main Color", Color) = (1,1,1,1)
     }
 
     SubShader
     {
         Tags
         {
             "SplatCount" = "4"
             "Queue" = "Geometry-100"
             "RenderType" = "Opaque"
         }
 
         Blend SrcAlpha OneMinusSrcAlpha
 
         CGPROGRAM
 
         #pragma surface surf Lambert
         #pragma target 4.0
 
         struct Input
         {
             float2 uv_Control : TEXCOORD0;
             float2 uv_Splat0 : TEXCOORD1;
             float2 uv_Splat1 : TEXCOORD2;
             float2 uv_Splat2 : TEXCOORD3;
             float2 uv_Splat3 : TEXCOORD4;
         };
 
         sampler2D _Control;
         sampler2D _Splat0,_Splat1,_Splat2,_Splat3;
 
         void surf(Input IN, inout SurfaceOutput o)
         {
             fixed4 splat_control = tex2D(_Control, IN.uv_Control);
             fixed4 firstSplat = tex2D(_Splat0, IN.uv_Splat0);
             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;
             o.Alpha = 1;
             if (tex2D(_Splat0, IN.uv_Splat0).a == 0)
                 o.Alpha = 1 - splat_control.r;
             else if (tex2D(_Splat1, IN.uv_Splat1).a == 0)
                 o.Alpha = 1 - splat_control.g;
             else if (tex2D(_Splat2, IN.uv_Splat2).a == 0)
                 o.Alpha = 1 - splat_control.b;
             else if (tex2D(_Splat3, IN.uv_Splat3).a == 0)
                 o.Alpha = 1 - splat_control.a;
         }
         ENDCG
     }
 
     Dependency "AddPassShader" = "Hidden/TerrainEngine/Splatmap/Lightmap-AddPass"
     Dependency "BaseMapShader" = "Diffuse"
 
     //Fallback to Diffuse
     Fallback "Diffuse"
 }