> I do follow the pattern of creating parent node and attributes first and
> creating child nodes next.

No. Before copying the attribute nodes you generate a child text node
with (a couple of newlines and a couple of tab characters)

  <xsl:template match="boot_object" xml:space="preserve">SSSSSS<advanced_boot_object>SS<!-- The error line is below-->SS<xsl:copy-of select="@*"/>

Normally the indentation in the stylesheet is ignored but you have asked
for it to be preserved (and copied to the output) by using xml:space and
so all those "S" (really whitespace characters) make text nodes.

You almost never want to use  xml:space="preserve" on an xsl namespace
element.

David