Hello,
a file
<?xml version="1.0" encoding="utf-8"?>
<xliff xmlns:mda="urn:oasis:names:tc:
xliff:metadata:2.0" xmlns="urn:oasis:names:tc:xliff:document:2.0" srcLang="en-US" version="2.0" >
<file id="1">
<group id="content">
<unit id="1">
<segment>
<source>some <pc id="p1" canDelete="no">sentence</pc></source>
</segment>
</unit>
</group>
</file>
</xliff>
passes XLIFF2.0 validation using Lynx.
When validated using xliff_core_2.1.sch (
https://tools.oasis-open.org/version-control/browse/wsvn/xliff/trunk/xliff-21/schemas/xliff_core_2.1.sch) an error "canDelete' attribute is set to 'no', but the corresponding element is missing in the sibling target." is reported.
Maybe the rule should check whether sibling <target> is defined first.