error on line 2 at column 6: XML declaration allowed only at start of document

Sitemap is not generating

http://www.goldengateparksegwaytours.com/sitemapindex.xml

This page contains the following errors:

error on line 2 at column 6: XML declaration allowed only at the start of the document
Below is a rendering of the page up to the first error.

https://wordpress.org/plugins/bwp-google-xml-sitemaps/