I am trying to send an invoice and I am recieving a very nasty error message, anyone know what the cause is?

Traceback (most recent call last): File "/opt/odoo-8.0-6/apps/odoo/lib/odoo-8.0_a2115ef-py2.7.egg/openerp/http.py", line 517, in _handle_exception return super(JsonRequest, self)._handle_exception(exception) File "/opt/odoo-8.0-6/apps/odoo/lib/odoo-8.0_a2115ef-py2.7.egg/openerp/http.py", line 538, in dispatch result = self._call_function(**self.params) File "/opt/odoo-8.0-6/apps/odoo/lib/odoo-8.0_a2115ef-py2.7.egg/openerp/http.py", line 294, in _call_function return checked_call(self.db, *args, **kwargs) File "/opt/odoo-8.0-6/apps/odoo/lib/odoo-8.0_a2115ef-py2.7.egg/openerp/service/model.py", line 113, in wrapper return f(dbname, *args, **kwargs) File "/opt/odoo-8.0-6/apps/odoo/lib/odoo-8.0_a2115ef-py2.7.egg/openerp/http.py", line 291, in checked_call return self.endpoint(*a, **kw) File "/opt/odoo-8.0-6/apps/odoo/lib/odoo-8.0_a2115ef-py2.7.egg/openerp/http.py", line 754, in __call__ return self.method(*args, **kw) File "/opt/odoo-8.0-6/apps/odoo/lib/odoo-8.0_a2115ef-py2.7.egg/openerp/http.py", line 387, in response_wrap response = f(*args, **kw) File "/opt/odoo-8.0-6/apps/odoo/lib/odoo-8.0_a2115ef-py2.7.egg/openerp/addons/web/controllers/main.py", line 949, in call_kw return self._call_kw(model, method, args, kwargs) File "/opt/odoo-8.0-6/apps/odoo/lib/odoo-8.0_a2115ef-py2.7.egg/openerp/addons/web/controllers/main.py", line 941, in _call_kw return getattr(request.registry.get(model), method)(request.cr, request.uid, *args, **kwargs) File "/opt/odoo-8.0-6/apps/odoo/lib/odoo-8.0_a2115ef-py2.7.egg/openerp/api.py", line 237, in wrapper return old_api(self, *args, **kwargs) File "/opt/odoo-8.0-6/apps/odoo/lib/odoo-8.0_a2115ef-py2.7.egg/openerp/addons/email_template/wizard/mail_compose_message.py", line 54, in default_get res.get('model'), res.get('res_id'), context=context File "/opt/odoo-8.0-6/apps/odoo/lib/odoo-8.0_a2115ef-py2.7.egg/openerp/api.py", line 237, in wrapper return old_api(self, *args, **kwargs) File "/opt/odoo-8.0-6/apps/odoo/lib/odoo-8.0_a2115ef-py2.7.egg/openerp/addons/email_template/wizard/mail_compose_message.py", line 103, in onchange_template_id values = self.generate_email_for_composer_batch(cr, uid, template_id, [res_id], context=context)[res_id] File "/opt/odoo-8.0-6/apps/odoo/lib/odoo-8.0_a2115ef-py2.7.egg/openerp/api.py", line 237, in wrapper return old_api(self, *args, **kwargs) File "/opt/odoo-8.0-6/apps/odoo/lib/odoo-8.0_a2115ef-py2.7.egg/openerp/addons/email_template/wizard/mail_compose_message.py", line 167, in generate_email_for_composer_batch template_values = self.pool.get('email.template').generate_email_batch(cr, uid, template_id, res_ids, fields=fields, context=ctx) File "/opt/odoo-8.0-6/apps/odoo/lib/odoo-8.0_a2115ef-py2.7.egg/openerp/api.py", line 237, in wrapper return old_api(self, *args, **kwargs) File "/opt/odoo-8.0-6/apps/odoo/lib/odoo-8.0_a2115ef-py2.7.egg/openerp/addons/email_template/email_template.py", line 515, in generate_email_batch result, format = self.pool['report'].get_pdf(cr, uid, [res_id], report_service, context=ctx), 'pdf' File "/opt/odoo-8.0-6/apps/odoo/lib/odoo-8.0_a2115ef-py2.7.egg/openerp/api.py", line 237, in wrapper return old_api(self, *args, **kwargs) File "/opt/odoo-8.0-6/apps/odoo/lib/odoo-8.0_a2115ef-py2.7.egg/openerp/addons/report/models/report.py", line 269, in get_pdf paperformat, specific_paperformat_args, save_in_attachment File "/opt/odoo-8.0-6/apps/odoo/lib/odoo-8.0_a2115ef-py2.7.egg/openerp/api.py", line 237, in wrapper return old_api(self, *args, **kwargs) File "/opt/odoo-8.0-6/apps/odoo/lib/odoo-8.0_a2115ef-py2.7.egg/openerp/addons/report/models/report.py", line 442, in _run_wkhtmltopdf wkhtmltopdf = [_get_wkhtmltopdf_bin()] + command_args + local_command_args File "/opt/odoo-8.0-6/apps/odoo/lib/odoo-8.0_a2115ef-py2.7.egg/openerp/addons/report/models/report.py", line 54, in _get_wkhtmltopdf_bin raise IOError IOError