I don't have administrator account anymore - macOS 10.13.4 I tried the answers on the similar previous question but nothing worked.
Obviously Apple removed the critical file .AppleSetupDone in the last Security Update.
Is there anybody who could help getting my administrator account back?
The removal of file .AppleSetupDone to trigger Setup Assistant is still here and fully functional in High Sierra, even in the latest 10.13.5 Beta. You can remove this file either in Single User mode or in Terminal when booted in Recovery Partition.

Another way to restore your admin account is to boot in Recovery Partition, starting Terminal and using the resetpassword command to launch the helper app (see attached screenshot).