In case you receive the error message
Unable to find a sitemap to generate
when trying to generate a Google Sitemap for an existing entry in Magento make sure to check that the sitemap ID generated is set correctly, i.e. not equal to 0 (zero).
In this particular Magento 1.9.1.0 setup the ID for the first sitemap wrongly was set to 0 on generation via Catalog / Google Sitemap, thus breaking the sitemap generation process through the Generate option: 
Correct Sitemap ID in Database
Manually setting the sitemap ID in the database to 1 for the first sitemap solved this problem:
Still, it’s strange that no error message was displayed when creating the first sitemap entry through Catalog / Google Sitemap.
Has anybody else experienced this behavior?
No. This way it is not solved. I checked file permission too. Any other alternate way?
Hi Roy,
did you check system.log and possible exception reports in var/report?
Please remove the website link from the comment.
Hello, the link was removed.
I had the exact same problem for my dashcams store. Magento was returning error saying “Unable To generate Sitemap” .
First of all i did database maintenance. you can google for a tutorial on how to run magento database maintenance.
2nd i went to System > Configuration > Developer and Enabled Log in Log settings.
This log file gets stored in var/log folder.
I tried to create sitemap again and error log was created. checked log and found that it was SQL error for on of extensions which i had already uninstalled.
I went to app/etc/modules . I opened .xml file for that extension and replaced word “true” with “false” . It switches off the extension.
i cleaned cached i.e. deleted everything inside var/cache folder.
then i tried sitemap generation again and it worked.
i didall this via cpanel.