Etrecheck report - failing hard drive. - can I fix?

Unfortunately, a failing hard drive cannot be fixed by the user. An Apple Store or Apple Authorized Service Provider (AASP) must take care of this, as this is a hardware repair. Go ahead and backup your data ASAP though, as your drive is failing. You can schedule an appointment here - Mac - Official Apple Support

Cheers,

Jack

Here are the install videos for your model - professional installation is recommended. Note: you have the default super painfully slow 5400 rpm hard drive installed; if you are going to have it replaced, at least get a 7200 rpm drive or an SSD. On the other hand, your Mac is going on 9 years old, so you will need to decide how much money you want to invest in an outdated machine.

https://eshop.macsales.com/installvideos/imac_21inch_late_2012/

Out of curiosity: are you running your Mac in Safe Mode regularly? It's meant to be a tool to isolate or fix issues.

https://support.apple.com/guide/mac-help/start-up-your-mac-in-safe-mode-mh21245/10.15/mac/10.15

Good luck! Hopefully you ordered one with something faster than a 5400 rpm spinning drive?