The Ubuntu 24.04 LTS is currently released for new installs only, the upgrades are not yet open, which is intentional & normal.
The Ubuntu Release team will assess the health of 24.04 LTS early next week (by convention) and decide when its ready for opening, yes in the past they’ve opened up the upgrades on the weekend after Thursday release; but usually it’s the next week (or a couple of weeks).
Your system is looking at what upgrades are available in this file (https://changelogs.ubuntu.com/meta-release) which if you look doesn’t include any entry of 24.04; as the upgrade is not currently available.
If you want the upgrade to be seen, you must also use the -d flag, which will cause your system to use the development file instead of the file you use without it, ie. this file (https://changelogs.ubuntu.com/meta-release-development) will be used where upgrades will be seen.
Note the “Supported: 0” there for 24.04; the could be seen as a warning, as upgrades are not yet open… for details on why, I suggest you view
and look especially at “Current blockers for enabling upgrades from Mantic” (mantic being the codename of 23.10).