Snapcraft clean syntax has changed
MODIFY
release.sh
release.sh
@@ -84,7 +84,7 @@ git push --tags
# the version of the built snap package will end up being `DATE_dirty`.
#
-snapcraft clean cordless -s pull
+snapcraft clean cordless
snapcraft
snapcraft push "cordless_${RELEASE_DATE}_amd64.snap"

Snapcraft clean syntax has changed