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>