I am using W3 Total Cache and I got:

Code:
 Set expires header 
Set the expires header to encourage browser caching of files.
Expires header lifetime:	 seconds
 Set cache control header 
Set pragma and cache-control headers to encourage browser caching of files.
Cache Control policy:	
 Set entity tag (eTag) 
Set the Etag header to encourage browser caching of files.
 Set W3 Total Cache header 
Set this header to assist in identifying optimized files.
 Enable HTTP (gzip) compression 
Reduce the download time for text-based files.
All those options selected. But I am still getting the issue with:

Code:
Specify a cache validator
I am not sure why this is happening and how to fix this?