The OS that can be updated is not displayed on the device
If the Software Update screen on the device does not show an OS that can be updated and displays the following message, OS updates are restricted by the Defer software updates setting in the Restrictions payload of the profile.
Your iPhone/iPad is running the latest software update allowed by your administrator.
When you want to update, check the release date of the target OS version and specify an appropriate number of deferral days.
https://support.apple.com/en-us/HT201222
Last updated: April 20, 2022