You can do this:
When you come to this section, some of the blogger will found difficulties to create the sitemap, you require to create a “sitemap.xml” file and “sitemap.xml.gz” file and upload it into the public_html directory.
How to create sitemap.xml and sitemap.xml.gz ?
If you do not know how to create “sitemap.xml” file and “sitemap.xml.gz”, simply open a notepad, save file as “sitemap.xml” and another one save file as “sitemap.xml.gz“
Upload it to ur webhosting public_html directory, you definitely found the same issue,
There was a problem writing your sitemap file. Make sure the file exists and is writable
There was a problem writing your zipped sitemap file. Make sure the file exists and is writable
The sitemap.xml and sitemap.xml.gz are not writable yet, to change the ftp file permission to become writable, click on the file, and edit the CHMOD. it require to be writable on this 2 file. You need to change the this the FTP file permission to become writable, or “777″.
Once done, both file are writable. You can update the option or rebuild sitemap manually