Error in my edge detection function?

Try This

c=imread('carplate1.jpg');
b=rgb2gray(c);
 
px=[-1 0 1;-1 0 1;-1 0 1];   %_horizontal edges,
b1=filter2(px,b);
figure,imshow(b1/255)
 
py=px';                      %%_vertical edges,
b2=filter2(py,b);
figure,imshow(b2/255)
 
edge_p=sqrt(b1.^2+b2.^2);
figure,imshow(edge_p/255)
%figure,imshow(edge_p/512), Dividing by 512 darkens the image 
%figure,imshow(edge_p/128), Dividing by 128 Lighten the image,
 
edge_t=im2bw(edge_p/255,0.3); %a binary image containing edges only can be produced by thresholding.
figure,imshow(edge_t)
%edge_g=im2bw(edge_p/255,0.3); 
%figure,imshow(edge_g)
 
%We can obtain edges by the Prewitt _filters
edge_k=edge(b,'prewitt');
figure,imshow(edge_k)
 
%We can obtain edges by the Prewitt _filters
edge_s=edge(b,'sobel');
figure,imshow(edge_s)