Consistent with its push to force the new “Passwords” app on their users, Apple has decided to block you from adding new “secure notes” to your keychain starting with macOS Sequoia. Which just adds one more annoyance to the ever growing list with recent macOS releases.
If you want to add secure notes anyway, you can do it on the Terminal:
security add-generic-password -a dummy -C note -D "secure note" -T "" -s "My secure note" -w "Dummy note" [keychain]Just make sure to not enter the actual note text on the console, as otherwise it will end up cleartext in your history file. Use Keychain Access to edit it.