The tag is present in Lintian version 2.104.232. That is the most recent version we know about.
There is no "Format" field in your .changes file. This probably indicates some serious problem with the file. Perhaps it's not actually a changes file, or it's not in the proper format, or it's PGP-signed twice.
Since Lintian was unable to parse this .changes file, any further checks on it were skipped.
Refer to Debian Policy Manual section 5.5 for details.