Update my ipad

Not all iPads are recent enough to be updated. The 4th generation iPad for example cannot be updated beyond iOS 10.3.4. If you are uncertain about which model you have, see: Identify your iPad model - Apple Support

You can't update to iOS 11 any more. Any device that supports iOS 11 also supports iOS 12, and iOS 12 is the only version available for those devices.

If you have an iPad 4, then it cannot run anything beyond iOS 10.3.4