Hello, nini2400!
One of my many notes can not be opened, the message says: unsupported note, this note uses unsupported features, to view or edit it, upgrade to the latest version of ios.
My iphone is updated to the latest ios version.
All my other notes are fine. Just this one note that can not open.
According to your post’s details, you have an iPhone 6s Plus running iOS 15. The reason that the message tells you to update, but Settings says that your iPhone is up to date, is because it needs you to upgrade to a newer version of iOS (e.g. iOS 16, iOS 17, or later), which your iPhone is too old to install (not having enough performance in your iPhone’s processor will cause issues)
A similar thing happened to me on my iPad with GarageBand, except I couldn’t run the app at all because my iPad is on iPadOS 16, and GarageBand now requires iPadOS 17 or newer. If that note is stored on iCloud, Gmail, or some other cloud service (refer to SravanKrA’s previous comment if you don’t know where it is stored), the reason the note can’t be accessed is probably because it was used on a device of yours that has a newer OS installed, and uses features that weren’t added on iOS 15.
If your Notes are stored on your iPhone and you have a separate Apple device signed in to the same Apple ID running a newer OS version, move the note to iCloud and try accessing it on the other device. If that doesn’t work or you don’t own a separate Apple device signed in to the same Apple ID running a newer OS version, then keep that note in iCloud and try accessing it from iCloud.com. If none of that works, contact the Official Apple Support. Maybe they can assist you in accessing your note.
I hope this helps!