My website has been online for a couple years. It's on the first result for Google but it doesn't seem to be indexed by Bing. I tried to fetch it with bing in their webmaster tools and it always returns 'Unexpected Error'. If I delete the robots.txt file the error is "cannot download robots.txt".
Bing has a help document that says the following about "Unexpected Error":
What does "Unexpected Error" mean?
If you are seeing frequent "Unexpected Error" results in Fetch as Bingbot, this may indicate that our crawler is not able to connect to your server and not getting a server response at all. Your server may be unintentionally blocking Bingbot based on user agent or IP range. To remedy this, look for server configurations or modules that may be dropping connections for Bingbot or work with your ISP to identify whether the issue is on their side.
Check your firewall. Your website host may also have a firewall that could be blocking the requests.