You need to declare the XLINK namespace in XSLT before you can reference it.
You can add it to the xsl:value-of element:
<xsl:value-of select="DCPType/HTTP/Get/OnlineResource/@xlink:href" xmlns:xlink="http://www.w3.org/1999/xlink" />
However, if you need to reference it in other areas of your stylesheet, then it would be easier to declare it at the top in your XSLT document element:
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="1.0" xmlns:xlink="http://www.w3.org/1999/xlink">
By the way, you do not need to use the same namespace prefix in the stylesheet as in your XML. The namespace prefix is ββused only as an abbreviation for the namespace URI. You can declare and use the XLINK namespace as follows:
<xsl:value-of select="DCPType/HTTP/Get/OnlineResource/@x-link:href" xmlns:x-link="http://www.w3.org/1999/xlink"/>
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