Try with /app/wildfly/bin/jboss-cli.sh --connect --command=":shutdown"
jboss-cli.sh --connect --command=:shutdown
should work as well.
It looks like this is caused by your automation tool. Apparently it provides a parameter '':shutdown'' which is not unterstood by the WildFly command line tool.
Which automation tool (version) are you using? Is there an option to create plain text instead of html?