Comment # 1 on bug 1202116 from Stefan Dirsch

This specific requirement has been introduced to fix boo#1201936. 
kernel-default-devel = 5.3.18-57 is available via 

 
http://download.opensuse.org/distribution/leap/15.3/repo/oss/x86_64/kernel-default-devel-5.3.18-57.3.x86_64.rpm

which needs kernel-devel = 5.3.18-57, which is available via

 
http://download.opensuse.org/distribution/leap/15.3/repo/oss/noarch/kernel-devel-5.3.18-57.3.noarch.rpm

Could it be, that you no longer have the original installation repository
enabled, which provide the initial versions of all needed packages? The output
of 

  zypper repos -u

would be very useful in this context.