Error installing SQL Server Database Engine Services Instance Features - The User Log directory in the registry is not valid
Ok I found this article

https://blog.sqlauthority.com/2015/05/16/sql-server-service-pack-error-the-user-data-directory-in-the-registry-is-not-valid-verify-defaultdata-key-under-the-instance-hive-points-to-a-valid-directory/

It says my CU update fails because of Default Log path does not exist anymore

Default Log path can be checked by one of below ways:


Execute T-SQL


exec [master].sys.xp_regread 
    @rootkey='HKEY_LOCAL_MACHINE', 
    @key='SOFTWARE\Microsoft\Microsoft SQL Server\MSSQL14.MSSQLSERVER\MSSQLServer\', 
    @value_name = 'DefaultLog'



SSMS -> Right Click Server -> Properties -> Database Settings -> Database Default Locations
Regedit -> HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Microsoft SQL Server\MSSQL14.MSSQLSERVER\MSSQLServer\DefaultLog


Updated Default Log path to point to existing location, and CU18 update installed fine without any errors