Can I use an old 21 inch iMac late 2012 running Mojave as a monitor for the Mac mini M2?

bala wrote:

I have an old 21 inch late 2012 running Mojave.

Can I use that monitor to run Mini Mac M2

No. That 2012 iMac supports Target Display Mode, using Thunderbolt input – but current Macs (including the M2 Mac mini) don't support using any iMac as a Target Display.

In addition, you'd need about $80 of specialized Thunderbolt connection hardware just to make a data connection. Given that you can get 24" 1080p monitors for that much (good ones for $150 or less) - and that you can get some 27" 4K monitors for $300 – $350, it wouldn't make much sense to sink that much money into cabling to reuse that old display even if that was an option.

bala wrote:

This request is from Australia

I have an old 21 inch late 2012 running Mojave.

Can I use that monitor to run Mini Mac M2

No because the M2 Mac mini does not support using the 2012 iMac as a display in Target Display Mode.

carefully read > Use your iMac as a display with target display mode - Apple Support

I am currently using 21 inch 2019 running Catalina.

Plus you should upgrade the 2019 iMac to a later macOS and use an aftermarket display, instead of holding it back just to use the 2012 iMac as a display.