Never seen that error before, and only google references seem to be proxy related. Appears to be an error message specific to Envoy which looks to be a component utilized by eBay.
Are you using a proxy, behind some sort of corporate firewall, and/or using the eBay app, and not a standard web browser?
If using the app, you need to take your query to ebay.com/ios or ebay.com/android whichever is applicable.
If not using the app, then more info would help to understand the issue.
