Update iOS 15.3

See if the following support article can help:

• If your iPhone, iPad, or iPod touch won't update - Apple Support

If any of the following are true, your phone may not be able to update:

• The phone is running an iOS beta. If so, then you will need to remove the beta profile first.
• The phone does not have enough free space. Typically it should have at least 5GBs available to update.
• The phone is configured for either or both Low Power Mode or Personal Hotspot.
• The phone is not connected to power and it has less than 50% charge left.
• The phone is not connected via Wi-Fi to the Internet. It won't update on a cellular-only connection.
• The phone is connected via a VPN