Did you turn on the cxml logs and see if you are getting response code 200 ?
if it is some other response code , it may be configuration error ..
if you are getting 200 response code still see this error then supplier site is asking for some credentials need to check with supplier..
hope this helps!
Hope that you are getting this specified error only to one Punch out Supplier.
Below point may help you to analyze the issue and identify root cause.
1) Punch out URL defined in CIF file should be correct. Login into Supplier Direct partition and extract the CIF from Catalog Manager. Get punch out URL setup in the system and get it verified with the vendor.
2) Follow below steps to troubleshoot any error code in punch out response
a) Switch ON application.cxml.punch out in debug mode
b) Create a requisition and click on punch out link
c) Capture the cxml logs, look for any error code in the response and take corrective action.
Note: Make sure to switch OFF the logs once done
3) Supplier site might be down. Coordinate with vendor for any issues with their punch out site.