Cannot get into my new iMac

• Make sure the keyboard is turned on
• Try clicking the mouse a couple times when the pointer is over the password dialog box.
• If still not working try plugging the Magic Keyboard (turned on) into the usb port using the charging cord.

After you have followed the above it may seem obvious but...

Wait a few seconds - the keyboard should connect - then start typing.

When setting up my M1 iMac I found that there was a short delay before the keyboard first responded - too quick to start typing and the first part of my password would be truncated. Once up and running no keypress delay.

If it’s new, return it for a refund! 

You can always buy another one! 😉

If it's new, it's under warranty. Contact Apple for it to be repaired.