I have a new iMac is luck in 5 minutes after setup how unlock it?

It doesn't matter if you used an old password or not; the Mac doesn't care. Whatever password you did use is the one you must now use to access your Mac.

If you can't remember the password you set, this is Apple's fix: https://support.apple.com/en-us/HT202860

Notice the last part:

If you can't reset your password

If you can't reset your Mac login password

Truly new Macs startup to the setup assistant; a series of maybe six questions to make your Mac usable.

Did your Mac start up to the set up screen? If not, it's a used Mac and you may never be able to use it. You may, but that depends on exactly what's going on.

What exact error message are you getting? Too many password attempts or what exactly? Is it the case your newly established password is just not working?