So, I got this same message also when trying to download the iOS 11 Beta pre-release through my MacOS. I read on another forum that you need to do this on the device that you want iOS on through the device.
Basically here is what I did.
1. Login to where you are trying to download the iOS you want on your iOS device.
2. Find the download link and then download it using your iOS device.
3. It will ask if you approve the use of the new certificate.
4. It made me restart.
5. Then iOS 11 beta was available for download through my iOS device in my device settings under Software Update.
Viola!
Hope this helps anyone having similar issues. Currently iOS11 is for developer accounts only but it sounds like the public is going to be able to download beta soon.