What is this error message with Google Sitemap plugin?

I enabled the google sitemap plugin – https://www.arnebrachhold.de/projects/wordpress-plugins/google-xml-sitemaps-generator/

but I keep getting these 2 error messages in my wordpress dashboard

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

Any ideas how to troublshoot?