I have ERPNEXT V12 when I press on print and PDF system Show this message 
Traceback (most recent call last):
File “/home/frappe/frappe-bench/apps/frappe/frappe/utils/pdf.py”, line 38, in get_pdf
filedata = pdfkit.from_string(html, False, options=options or {})
File “/home/frappe/frappe-bench/env/lib/python3.6/site-packages/pdfkit/api.py”, line 72, in from_string
return r.to_pdf(output_path)
File “/home/frappe/frappe-bench/env/lib/python3.6/site-packages/pdfkit/pdfkit.py”, line 156, in to_pdf
raise IOError(‘wkhtmltopdf reported an error:\n’ + stderr)
OSError: wkhtmltopdf reported an error:
Exit with code 1 due to network error: ContentNotFoundError
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File “/home/frappe/frappe-bench/apps/frappe/frappe/app.py”, line 60, in application
response = frappe.api.handle()
File “/home/frappe/frappe-bench/apps/frappe/frappe/api.py”, line 55, in handle
return frappe.handler.handle()
File “/home/frappe/frappe-bench/apps/frappe/frappe/handler.py”, line 22, in handle
data = execute_cmd(cmd)
File “/home/frappe/frappe-bench/apps/frappe/frappe/handler.py”, line 61, in execute_cmd
return frappe.call(method, **frappe.form_dict)
File “/home/frappe/frappe-bench/apps/frappe/frappe/init.py”, line 1042, in call
return fn(*args, **newargs)
File “/home/frappe/frappe-bench/apps/frappe/frappe/utils/print_format.py”, line 92, in download_pdf
frappe.local.response.filecontent = get_pdf(html)
File “/home/frappe/frappe-bench/apps/frappe/frappe/utils/pdf.py”, line 46, in get_pdf
frappe.throw(_(“PDF generation failed because of broken image links”))
File “/home/frappe/frappe-bench/apps/frappe/frappe/init.py”, line 364, in throw
msgprint(msg, raise_exception=exc, title=title, indicator=‘red’)
File “/home/frappe/frappe-bench/apps/frappe/frappe/init.py”, line 350, in msgprint
_raise_exception()
File “/home/frappe/frappe-bench/apps/frappe/frappe/init.py”, line 316, in _raise_exception
raise raise_exception(msg)
frappe.exceptions.ValidationError: PDF generation failed because of broken image links