Light direction based Lerp

I'm new to CG [first shader actually] but I want to lerp between two textures based the light direction.

As it is I've tried to use a dot product of the light vector and surface normal to try and create the interpolator for the lerp below

the code - Shader "DS/LightDirBasedLerp" {

     Properties 
     {
         _MainTex ("Base (RGB)", 2D) = "white" {}
         _Ramp2D ("Gradient Ramp",2D) = "gray" {}
         _RampColor("Ramp color", Color) = (.5,.5,.5,1)
         _DiffLit("Lit Diffuse", 2D) = "white" {}
         _DiffUnlit("Unlit Diffuse", 2D) = "black" {}
         
     }
     
     
     SubShader {
         Tags { "RenderType"="Opaque" }
         LOD 200
         
         CGPROGRAM
         #pragma surface surf Ramp
         #pragma target 3.0
 
         sampler2D _MainTex;
         sampler2D _Ramp2D;
         float4 _RampColor;
         sampler2D _DiffLit;
         sampler2D _DiffUnlit;
 
         struct Input 
         {
             float2 uv_MainTex;
         };
         
         half4 LightingRamp (SurfaceOutput s, half3 lightDir, half3 viewDir, half atten)
         {
             float NdotL = dot(s.Normal, lightDir);
             float NdotE = dot(s.Normal, viewDir);
         
             
             
             float diff = (NdotL * 0.3) + 0.5;
             float2 gradRampUV = float2(NdotE* 0.8, diff);
             float3 gradRamp = tex2D(_Ramp2D, gradRampUV.xy).rgb;
             float3 lerpedTex = lerp(_DiffLit, _DiffUnlit, gradRamp.x);
             
             float4 c;
             c.rgb = lerpedTex;
             c.a =  s.Alpha;
             return c;
         }
 
         void surf (Input IN, inout SurfaceOutput o) 
         {
             half4 c = _RampColor;
             o.Albedo = c.rgb;
             o.Alpha = c.a;
         }
         ENDCG
     } 
     FallBack "Diffuse"
 }

issue being I'm getting an error on line 44 saying - 'unable to find compatible overloaded function "lerp(sampler2D, sampler2D, Float)'

                 float3 lerpedTex = lerp(_DiffLit, _DiffUnlit, gradRamp.x);

as far as I know that's fine, right? anyone know the solution?