Debuginfo packages are in separate repos, see dnf repolist --all *debuginfo. You should probably make sure you don’t have any F36 repos still enabled.
Easy way
Use debuginfo-install from dnf-plugins-core:
sudo dnf debuginfo-install kernel
Manual way
Enable repo(s) for one command only:
sudo dnf --enablerepo=fedora-debuginfo,updates-debuginfo install kernel-debuginfo
Enable repo(s) permanently (probably not what you want):
sudo dnf config-manager --set-enabled fedora-debuginfo updates-debuginfo
Disable repo(s):
sudo dnf config-manager --set-disabled fedora-debuginfo updates-debuginfo