Indeed. http://download.opensuse.org/distribution/leap/15.3/repo/oss/ repo is enabled and I have no idea, why this version is not found by zypper/YaST. Of course you can download both packages manually and install them via zypper in \ kernel-default-devel-5.3.18-57.3.x86_64.rpm kernel-devel-5.3.18-57.3.noarch.rpm before doing the nvidia driver update.