6.15.4 has been superceded by the 6.15.5 kernel and is no longer in the fedora updates repo. You may be able to reinstall it by using sudo dnf reinstall kernel*6.15.4* --enablerepo updates-archive
If you just do a new update it should install the newer kernel for you.
How did you remove that kernel?
Did you manually delete files (which can easily cause problems) or did you use either dnf or the package manager to remove it?
If you used dnf or the package manager then it should not show up in grub, but if you manually deleted files then it may still show in grub and will likely be very broken. Removing it completely and cleanly would involve a reinstall shown above then use dnf to remove it.