Transform an XML by grouping the fields - xslt

Question

My knowledge to XSLT is limited but always eager to learn. I am currently working on a template that requires to transform the XML input. I've been trying to group the InvoiceNum fields and not getting anywhere.
I am getting an error: Envision.Utilities.XsltEngine-Object reference not set to an instance of an object.
Here's the input XML for reference:
<?xml version='1.0' ?>
<Request>
<Information>
<ImageID>987456321</ImageID>
<Contract>123456789</Contract>
<Lastname>MICKEYMOUSE</Lastname>
</Information>
<Document>
<InvoiceNum>123456823</InvoiceNum>
<Reference>AD20985224</Reference>
<InvoiceNum>100000123</InvoiceNum>
<Reference>AS20101387</Reference>
<InvoiceNum>858511825</InvoiceNum>
<Reference>GF96844</Reference>
<InvoiceNum>885154145</InvoiceNum>
<Reference>FGFD2018</Reference>
<InvoiceNum>25241111</InvoiceNum>
<Reference>SD88888</Reference>
<InvoiceNum>8571414</InvoiceNum>
<Reference>DF864841254</Reference>
</Document>
</Request>
Here's my XSLT format for reference:
What am I missing? Is there better way to format the XSLT template I have currently below? Any help is greatly appreciated.
<xsl:stylesheet version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output method="xml" version="1.0" encoding="UTF-8" indent="yes"/>
<xsl:strip-space elements="*"/>
<xsl:param name="cols" select="3" />
<xsl:template match="Request">
<table border="1">
<xsl:apply-templates select="InvoiceNum[position() mod $cols = 1]"/>
</table>
</xsl:template>
<xsl:template match="Document">
<xsl:variable name="group" select=". | following-sibling::InvoiceNum
[position() < $cols]" />
<xsl:for-each select="*">
<xsl:variable name="i" select="position()" />
<Invoice>
<InvoiceNumber>
<xsl:value-of select="InvoiceNum()"/>
</InvoiceNumber>
<xsl:for-each select="$group">
<xsl:value-of select="*[$i]"/>
</xsl:for-each>
</Invoice>
</xsl:for-each>
</xsl:template>
</xsl:stylesheet>
Here's the XML output I'd like to have:
<InvCase>
<Invoices>
<InvoicemNumber>InvoiceNum1</InvoicemNumber>
<InvoicemNumber>InvoiceNum2</InvoicemNumber>
<InvoicemNumber>InvoiceNum3</InvoicemNumber>
</Invoices>
</InvCase>

Answer 1

It looks like you are trying to group in the InvoiceNum into groups of 3. The first issue you have is that in your template matching Request you do this...
<xsl:apply-templates select="InvoiceNum[position() mod $cols = 1]"/>
But InvoiceNum is not a child of Request, and so that selects nothing. You probably need to do this...
Additionally, you have a template matching Document, but this probably needs to match InvoiceNum (Doing following-sibling::InvoiceNum would not return anything if you were matching Document as the InvoiceNum elements are children of Document not following siblings).
Try this XSLT
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output method="xml" version="1.0" encoding="UTF-8" indent="yes"/>
<xsl:strip-space elements="*"/>
<xsl:param name="cols" select="3" />
<xsl:template match="Request">
<InvCases>
<xsl:apply-templates select="Document/InvoiceNum[position() mod $cols = 1]"/>
</InvCases>
</xsl:template>
<xsl:template match="InvoiceNum">
<xsl:variable name="group" select=". | following-sibling::InvoiceNum[position() < $cols]" />
<Invoice>
<xsl:for-each select="$group">
<InvoiceNumber>
<xsl:value-of select="."/>
</InvoiceNumber>
</xsl:for-each>
</Invoice>
</xsl:template>
</xsl:stylesheet>