Magic Keyboard and iMac mid 2010

If you've never paired the keyboard to the Mac (by temporarily plugging in a cable between the two), you will need to do that. (But first verify that you're running a version of macOS that's recent enough to support the keyboard.)

Set up your Magic Keyboard, Magic Mouse, or Magic Trackpad with your Mac - Apple Support

There are several varieties of Magic Keyboard, each with its own system requirements.

I'm using a "Magic Keyboard with Numeric Keypad – US English" that requires a "Bluetooth-enabled Mac computer with macOS 10.12.4 or later." But the version without the keypad requires macOS 11.3 or later. The ones with Touch ID require Apple-Silicon-based Macs and macOS 11.4 or later.

So, which Magic Keyboard are you using? And which version of macOS Sierra 10.12? It sounds as though you need at least the 10.12.4 update.

Has it worked before?