Symptom

When saving a schema or previewing a form you get an error such as this one:

Cause

Generally the problem is that your XPath is wrong. In this particular case, the XPath is referring to the extension schema namespace rather than to the correct namespace, i.e. nms:recipient.

Fix

Correct your XPath, the syntax in a transaction schema for a buyer link would be:

<element label="buyer" name="buyer" target="nms:recipient" type="link"/>

even though the Recipients schema might have been extended.

Note that the link to the product in this transaction schema does use the custom namespace, because neo:product is a new datatype, not an extension:

<element label="product" name="product" target="neo:product" type="link"/>

Preventative

Review the XPath rules, and remember that an extension schema is not a new datatype. Rather it is a way to keep schema modifications isolated so they are not lost during upgrades.