I don’t think a plain vanilla install of 8.6 would have this repository:
Failed to download metadata for repo 'pgdg15': repomd.xml GPG signature verification error: Bad GPG signature
That’s a PostgreSQL repository, so a third-party repository. That one does not have a good GPG signature it seems. Try disabling that repository and running:
dnf upgrade
And see if the error still shows in the logs.