If you got the above message when trying to implement the XML sitemap, then you have the solution here.
I’m not touching the plugin installation part here as it is very well described by the plugin owner arne brachhold.
‘Make sure the file exists and is writable’. If we go by the sentence, we clearly understand that some file is missing. You may think of downloading the plugin again but it wont solve your problem.
When the plugin creates a sitemap, it should have some file to write it on right? To provide the plugin the file it needs just use your FTP.
Open your FTP and create a new file on the server side in the root directory and name it as sitemap.xml. Create another file similarly and name it as sitemap.xml.gz which will be created in zipped (compressed) format. This compressed file is optional and you can uncheck this option in the plugin settings to just use one sitemap.
Some FTP’s like filezilla may not have the option of making a new folder on server side. In such cases simply open your notepad on your desktop and make an empty file and name it as sitemap.xml. Similarly make an empty file and name it as sitemap.xml.gz. Then transfer them to your server using the FTP into the root directory (public_html) of your site. Make sure they are writable (normally, a 644 CHMOD should work; if it doesn’t, change it to 666 and try changing it back to 644 after you’ve built the sitemap since 666 is not safe).
After you’ve done that come back to settings and click on XML-sitemap. You will again see two red coloured lines in the beginning but hold your patience and click on build your sitemap manually.
There you go, your sitemap has been successfully built, wasn’t it?
Now everything that needs to be done using the FTP is done. You can now tinker with any setting in the plugin settings page as you like.
If your problem wasn’t solved even after this then do not hesitate to put a comment below.