Cannot create Plan in sandbox 6.0.1

The URL you provide in the question is to your user object ... a plan must be created against a project area, not a user object.   Once you have a project area, you would need to have permission to create a plan in that project area.   If you are using a jazz.net sandbox project area, then you would either need to be the creator of that project area, or you would need the creator to add you as a member of that project area.