I had this problem by myself while developing a plugin.
I think you were using the sonar--plugin-archetype to create your surrounding. If you do so your whole project is under GNU 3 license and a header within every class is expected telling this. The predefined pom.xml contains a part where this is defined. Search for "license" in your pom.xml and delete this part.
If this is not fixing you problem just add -Dlicense.skip=true
to your maven goal.
The expected header is like (please notice that the first things are set while you use -sonar-archetype)
/*
* MyLanguage Plugin
* Copyright (C) MyYear MyCompany
* dev@sonar.codehaus.org
*
* This program is free software; you can redistribute it and/or
* modify it under the terms of the GNU Lesser General Public
* License as published by the Free Software Foundation; either
* version 3 of the License, or (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
* Lesser General Public License for more details.
*
* You should have received a copy of the GNU Lesser General Public
* License along with this program; if not, write to the Free Software
* Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02
*/