Printout from; sudo dnf -v system-upgrade download --releasever=38
Loaded plugins: builddep, changelog, config-manager, copr, debug, debuginfo-install, download, generate_completion_cache, groups-manager, needs-restarting, playground, repoclosure, repodiff, repograph, repomanage, reposync, system-upgrade
DNF version: 4.15.0
cachedir: /var/cache/dnf
Before you continue ensure that your system is fully upgraded by running "dnf --refresh upgrade". Do you want to continue [y/N]:
I hit; y
After going thru 2730 packages, (already installed)
Came the following;
Total 3.6 MB/s | 168 MB 00:46
Using rpmkeys executable at /usr/bin/rpmkeys to verify signatures
Problem opening package FlightGear-data-2020.3.18-1.fc38.noarch.rpm
The downloaded packages were saved in cache until the next successful transaction.
You can remove cached packages by executing 'dnf clean packages'.
Error: GPG check FAILED
[frank@asus-fedora ~]$ sudo dnf clean packages
[sudo] password for frank:
151 files removed
[frank@asus-fedora ~]$ sudo dnf system-upgrade reboot
Error: system is not ready for upgrade
[frank@asus-fedora ~]$ sudo dnf -v system-upgrade reboot
Loaded plugins: builddep, changelog, config-manager, copr, debug, debuginfo-install, download, generate_completion_cache, groups-manager, needs-restarting, playground, repoclosure, repodiff, repograph, repomanage, reposync, system-upgrade
DNF version: 4.15.0
cachedir: /var/cache/dnf
Error: system is not ready for upgrade
[frank@asus-fedora ~]$
Does this help you any to see the problem?