Problem with "System Administrator"

If it is locked by a Firmware Update, then you would see:

Lock icon

If your Mac is using a firmware password, the lock icon appears when you try to start up from another disk or volume, such as an external drive or macOS Recovery. Enter the firmware password to continue.

If it has been locked with a PIN code, then you would see:

System lock PIN code

Your Mac asks for a PIN code when it has been remotely locked using Find My. Enter the four-digit or six-digit passcode to continue.

Either of these screens would appear when you boot the computer. You would need to take the computer to an Apple shop for help. However, you must be able to prove you are the original purchaser.

Here's what you need to do:

Internet/Network Recovery of macOS

     This will perform a factory reset.

1. Restart the computer. Immediately, at or before the chime, hold down the (Command-Option-Shift-R) keys until a globe appears.
2. The Utility Menu will appear in from 5-20 minutes. Be patient.
3. Select Disk Utility and click on the Continue button.
4. When Disk Utility loads select the target drive (out-dented entry w/type and size) from the side list.
5. Click on the Erase button in Disk Utility's toolbar. A panel will drop down.
6. Set the partition scheme to GUID.
7. Set the Format type to APFS (SSDs, only if installing Mojave or Catalina ) or Mac OS Extended, (Journaled.)
8. Provide a volume name, usually "Macintosh HD."
9. Click on the Apply button, then click on the Done button when it activates.
10. Quit Disk Utility and return to the Utility Menu.
11. Select Install OS X and click on the Continue button.

Once this has successfully completed, you can upgrade to Catalina: How to upgrade to macOS Catalina.

If you perform the steps outlined earlier, then this will erase the drive and download macOS from Apple's servers. If this isn't working for some reason, then you need to provide more detail about what you are doing. I am not able to read anything in the images you posted because I cannot read/write your language.

You said that you have pre-installed macOS. If you still can't install macOS via Internet Recovery, then I don't know what the problem is. Perhaps the computer is locked by a Firmware Password but you haven't said that was the case.

OK, then you are all set, right?