Запуск iMac

That means a firmware lock has been enabled on the computer. The only way to get past it is by providing the proper password. In order to completely remove the firmware lock requires knowing the correct password or by providing proper valid acceptable proof of purchase & personal ID to Apple so that Apple can remove the firmware lock.

If you don't have any of these things, then there is nothing you can do about it. If the internal drive still contains a valid OS installation, then it should continue booting to the internal drive. However, it is impossible to boot from external media without knowing the firmware lock password.

Here is an Apple article about the firmware lock:

Set a firmware password on your Mac - Apple Support