How to reset forgotten screen time passcode on older iPad with iOS 12.5.5 and says iOS is up to date / cannot be updated with later versions

If you have forgotten the ScreenTime passcode, one of the two procedures outlined within this link should resolve the problem:

If you forgot your Screen Time passcode - Apple Support

An iPad capable of supporting iPadOS 13.x should be updated to the most current version before attempting to resolve a forgotten ScreenTime passcode. The most recent iPadOS updates have introduced a new mechanism whereby a forgotten ScreenTime passcode can be reset without wiping the iPad. Unfortunately, for your iPad Air1, mini2 or mini3, this is not an option.

To reset a forgotten ScreenTime passcode on older versions of iOS/iPadOS, the iPad must be reset “as new”, losing all locally stored data. In this situation, you’ll not be able to restore your data from an iCloud or iTunes backup - as the forgotten passcode is integral to the backup.

I hope this information is helpful to you in successfully resetting your forgotten passcode.