IOS 11 Update

What model of iPad do you have? Identify your iPad model - Apple Support

iOS 11 is no longer available to update to for any device that supports it since any device that can support iOS 11 can also support iOS 12, and you can only update to the most of to date version of iOS your device supports.

If you have an iPad 4 or earlier, then it cannot be updated to iOS 11 or 12.

What model iPad do you have and what is it's current iOS level? It's highly likely if it has been stuck at either iOS 9.3.5 or 10.3.3, it is too old and incapable of being updated to the current iOS 12.4. You couldn't update to iOS 11 anyway as you'd have to go to iOS is 12.4 if your device was capable of updating.

Writing an effective Apple Support Communities question - https://discussions.apple.com/docs/DOC-5931