Ensure that you are using the correct url for WebPurchasing by locating it in the WebPurchasing Configuration Tool
1.  From Start, go to All Programs, The Financial Edge, Webportal, Configuration Tool.
2.  On the Summary page, the URL for Webpurchasing is located to the right of the Website link at the top of the page.

If you are using a redirected URL for WebPurchasing, then contact your network administrator to obtain the correct redirected url.