Trying to install catalina on my late 2012 imac but my HD is greyed out. Help appreciated.

First, since Time Machine is on, I would run a backup first! Your Mac hasn't been backed up since 2020.

After a backup, follow these steps:

1. Shut down the Mac.
2. Turn on the Mac, and immediately press and hold Option + Command + R until you see a spinning globe.
3. Once you are in macOS Recovery, verify you are connected to WiFi in the top right corner. If possible, connect directly to the internet with Ethernet. 
4. Go to the reinstall macOS app.
5. Follow the prompts, and install on Macintosh HD.

Jack

Welcome to the Apple Support Communities.

If you hover over the grayed out Macintosh HD does it say why you can’t install there?

Jack

dc_in_nc wrote:

Yes, it says This disk is used for Time Machine backups

Is it being used for Time Machine? What are you booting from?

You're welcome!

Makes sense. A good backup drive would be a G-Technology 4TB G-DRIVE mobile USB-C Portable Hard Drive - Apple.

Is that a screenshot from your Mac? If so, you are using Time Machine, but Automatic backups are off.