There are several different reasons you might receive a 501 response when creating a co-citation feed using RSS Authority Sniper.
Improperly Formatted RSS Feeds
The most common reason is that there is something wrong with one of the companion feeds you submitted. If the feed is malformed in some way, sometimes the RSS parser won't be able to read it.
So you should make sure that all items in the companion feeds you chose really are valid, properly formatted RSS feeds, (Do not add plain HTML pages in the companion feeds as that is not the correct place for them.)
You can use a feed validator to check if the RSS feed you want to use is properly formatted.
Sometimes if there is a companion feed that just won't be accepted, you might need to change the companion feeds you choose.
Slow Companion Feed Websites
Another possible reason is that the request timed out waiting for one of the site that owns the companion feed to answer with their content feed.
In the case of timeouts, sometimes the co-citation feed actually is created, but finishes too late for the RSS Authority Sniper software to receive the reply.
You can check this by logging into the web dashboard of Rankfeedr at http://app.rankfeedr.com and looking to see if the feed was created or not.
If the feed was properly created, then you can simply take the feed URL bring it back to RSS Authority Sniper, where you would place it in the Rankfeedr URL box on tab 4.
Set The API Timeout in the Software to Max
One last thing you should do:
Go into the Settings tab of RSS Authority Sniper and make sure the API Timeout setting is at the max value. This will cause the software to wait longer for a reply.