Unable to download iOS 18 on iPhone even with sufficient storage

What happens when you try? You may only be able to download iOS 26.1.

Can you post a screenshot of the whole screen, it could reveal why you aren't able to update.

Make sure you:

• Are using a WiFi connection (not a hotspot).

• Are not using a VPN connection.

• Have your battery charged at least 50%. Better yet, keep it plugged in during the update.

• Do not have power save mode turned on.

• Do not have a beta version installed.

• You have at least 10 gigs of storage available.

Click on this link on what to do if you can't update your phone -

If your iPhone or iPad won't update – Apple Support (UK)

Your tag line says that you have an iPhone 12 running iOS 15. If that is the case, iOS 26 might be the only major update still available. Also, you might need the help of a Mac or a Windows PC to jump up by several iOS versions at one time.

For clarity, I assume your device is older and not eligible for update to iOS 26? If eligible for iOS 26, you can't install iOS 18 as it is not signed anymore. (all newer than A12 chip can't install iOS 18 anymore)