I have a ipad mini 2 that is on ios 9.3.5 and I want to update it to ios 11 but there has not been a ios 11 update available for it in a long time. What do I do?

Are you sure it’s an iPad mini 2, and not a first generation iPad mini? While the 2 can run iOS 11 (which is confirmed here https://www.apple.com/ios/ios-11/), the first generation can only be updated to iOS 9.3.5.

Check your model https://support.apple.com/en-us/HT201471 and if it is indeed a mini 2, you can troubleshoot the issue, but we need more info on how you’re trying to update, and what happens when you do. I’d start here though: https://support.apple.com/en-us/HT204204