Can't update to iOS 26 on my iPhone

Since you mention the beta – is the iPhone 12 currently running a beta? If so, this may apply:

How to uninstall iOS or iPadOS beta software - Apple Support

"To remove the iOS or iPadOS beta and install the current publicly released version of iOS or iPadOS, you need to erase and restore your device.

If you have an archived backup, after erasing and restoring your device, you can set it up from that backup."

Having the same problem. Wanted to update my iPhone 11 Pro Max, and iPhone 11. Didn't have enough space to do it wireless on either, so plugged them into my MacBook. Upgraded to iOS 26 fine on one. On the other I updated to iOS 18.7 first. After doing so, the option to upgrade to 26 doesn't show on the MacBook Air. Scrolled entire page. Does show on the iPhone, but when you click it, it only tells you there's not enough available gigs to do it wirelessly.

elcoz wrote:

After doing so, the option to upgrade to 26 doesn't show on the MacBook Air.

Does your MacBook Air have an Apple Silicon processor – a M1, M2, M3, or M4 chip? If so, when you have the Software Update panel open, try pressing Command-R to get the Mac to refresh the information shown. That trick is something I didn't know about until someone else mentioned it in another thread here.

If you have an Intel-based MacBook Air, the reason that you are not seeing the macOS 26 Tahoe update is that Apple is not offering it for any Intel-based MacBook Air.

Is there a bar at the far bottom of your screen showing "also avaliable upgrade to iOS 26"

My iPhone XR still won’t update past iOS 18.7

You are correct. It never will. Apple explains here:

Why I can’t download on iOS 26

Not worth the download until they figure out the issues! Wish I could go back to iOS 18