XML Sitemap generate giving error - Uncaught exception 'XMLSitemapGenerationException' with message '

I have installed XML Sitemap 6.x-2.0-rc2, when i click on Rebuilding Sitemap then giving error
An error occurred. /batch?id=6&op=do <br /> <b>Fatal error</b>: Uncaught exception 'XMLSitemapGenerationException' with message 'Could not open file

When i check report log then it show

Cron run exceeded the time limit and was aborted

I am using Drupal 6.28 version.

Trying to rebuild sitmapxml file, the batch proccess never ends successfully.