To activate Data Storage option in WinCC Unified v18 u have to do following things.
1. Install SQL Server 2019. Create WINCCUNIFIED instance.
2. Next using SSMS In folder Security\Logins create new user NT SERVICE\WCCILScsService. I selected server roles "public" and "sysadmin".
3. In runtime settings of Wincc Unified select "Microsoft SQL" in Storage system.
I can confirm that the steps still working for V20.
SQL server 2022 STD is needed for V20, which can be get from the WinCC Runtime Professional iso, see Siemens supports site.
Install it from the SQL install folder, don't use the Siemens setup, because it will be create a WINCC instance instead of WINCCUNIFIED.
Addition you have to run (as admin) the WinCC runtime configurator to get the MS SQL database detected.