If you are encountering the error "Kernel headers not found for target kernel" while installing VirtualBox Guest Additions, the solution to resolve the issue is by installing kernel-devel package.

Open terminal and run command uname -r to list current linux kernel version:

uname -r

The output is similar to:

6.2.9-300.fc38.x86_64

Then install the kernel-devel package which matches the current linux kernel version:

sudo dnf install kernel-devel-6.2.9-300.fc38

When the installation is complete, go to directory where VirtualBox Guest Additions image file is mounted.

cd /run/media/$USER/VBox_GAs_7.0.8/

Then install the VirtualBox Guest Additions again

sudo ./VBoxLinuxAdditions.run

The error should disappear now.

Then restart your system.

If the Fedora guest still doesn't fit screen, try to resize the window.