This means that the URL you entered in the web part’s Site URL field can not be understood, or that your blueprint is not formatted correctly. To correct your Site URL, please go to the Site URL field and double-check how you specified the URL. The format of the URL must be either http://Site-URL or http://Site-URL/Sub-Site-Name, and must be a valid URL.
To correct your blueprint, make sure it is formatted as follows, and that the referenced site template exists.
<?xml version="1.0" ?>
<Workplaces>
<Workplace id="ID NAME">
<Workspace>
<Title>Title</Title>
<Description>Workplace
Description</Description>
<URL>/Workplacename</URL>
<Template>template file name</Template>
<Language>1033</Language>
</Workspace>
</Workplace>
</Workplaces>