The format box is currently forced, which is why you cannot uncheck it.
A problem was discovered during Quality Assurance testing for Ubuntu noble (24.04), where insufficient time was available for it to be fixed, and corrected prior to release.
https://bugs.launchpad.net/ubuntu-desktop-provision/+bug/2058638
The workaround implemented in the time available was to force format, thus your inability to uncheck format.
There are plans to correct this issue when time allows, and being that ubuntu-desktop-installer is a snap package, it will be available once released, but no time line on when the ubuntu-desktop-provision fix will be available is currently known.
It impacts the ubuntu-desktop-installer, thus affects Ubuntu Desktop and all flavors of 24.04 using ubuntu-desktop-installer.
It does not impact Lubuntu, Kubuntu or Ubuntu Unity 24.04, as they use the calamares installer.
The current workaround, is to format the partition on install, then post-install, restore your data from backups.
The other alternative is using another ISO that uses a different installer, then alter the installed packages so as to achieve what you want (ie. install Lubuntu 24.04 for example, then adjust what is installed to make it what you want); alas this is dirty & maybe more work than restoring backups.
ps: I've assumed you have backups, but if you don't, stop now and boot live media (eg. 24.04 and use the TRY mode) and create backups of your data now !!