The tag is present in Lintian version 2.104.244. That is the most recent version we know about.
The keyword value of the "Urgency" field in the .changes file is not
one of the allowed values of low, medium, high, critical, and emergency
(case-insensitive). This value normally taken from the first line of the
most recent entry in
debian/changelog, which is probably where
the error is.
Refer to Debian Policy Manual section 5.6.17 for details.