Unable to install iOS 15.6 on iPad Air 2 and iPhone 6s

The support article you came here from should provide you with the steps needed to try to resolve this issue. Have you tried them all yet? If so, and they didn't help, I suggest that you contact Apple Support directly. To do so, simply select the "Get Support" link in the upper right-hand side of this forum's web page.

Make sure you have enough storage on either device.

What happens when you try?