I got this error when doing custom XSLT with a BizTalk map.
It was a case of “brain fog”. The proper keyword is xsl:choose, not xsl:case. Other languages call this a switch or a case. I just temporarily forgot that XSLT calls it a “choose”.
Example of what the xsl:choose looks like (from https://www.w3schools.com/xml/xsl_choose.asp
<pre> <xsl:choose> <xsl:when test="expression"> ... some output ... </xsl:when> <xsl:otherwise> ... some output .... </xsl:otherwise> </xsl:choose> </pre>