If you want an easy release-upgrade path, you need to release-upgrade BEFORE the release goes EOL, or very soon after that.
There is no 24.x LTS version; please be specific with details; as they do matter.
The instructions you linked, apply to releases that weren’t EOL, or within a couple of days of EOL, as if you read the EOL notices (here as example) there is a note that says
as there are extra steps required AFTER that occurs; which you’re now finding.
The extra steps are covered in EOLUpgrades - Community Help Wiki which tell you what is required for a release that has gone EOL.
Ubuntu 23.04 had a single upgrade path, which was to Ubuntu 23.10 or the next release, however that release is EOL now too. The upgrade instructions found at ManticUpgrades - Community Help Wiki assume that 23.10 isn’t EOL (which it now is).
You need to follow the EOLUpgrades links details to ensure you system is fully upgraded, THEN apply the instructions in the “Unsupported upgrades” step that should allow you to release-upgrade to Ubuntu 23.10.
Once on Ubuntu 23.10, you can then repeat the process and release-upgrade to Ubuntu 24.04 LTS (NobleUpgrades - Community Help Wiki) which is the easier step, as 24.04 is a supported release.
Your alternative is to backup your data, and install a supported release… You’ve left it so late, and now have more manual steps to achieve your required release-upgrades, a backup, install & restore of data is likely far faster/easier.
In future please remember 23.04 tells you it’s the 2023-April release, making it’s 9 months of supported life pretty easy to calculate (ie. Jan 2024), and the supported release-upgrade path only remains open for six months after that date (where the next release is not an LTS; ie. you had till 23.10 reached EOL in July 2024).