Namespace prefix is not defined

An element or attribute uses a prefix (inv:) without its xmlns:inv declaration.

The message, as the validator reports it

Namespace prefix inv on invoice is not defined

Why it happens

A fragment cut out of a larger document, where the namespace was declared on an ancestor.
A misspelt prefix.

Declare the prefix on the element or an ancestor: xmlns:inv="urn:example:invoice", with the namespace URI the schema expects.

Reports the error

<inv:invoice/>

Fixed

<inv:invoice xmlns:inv="urn:example:invoice"/>