can't do a software update my mac

Your tag line says that you have a 21.5" iMac running High Sierra. What does it say about the model year of your iMac if you select About This Mac from the Apple () menu?

If it says

• Late 2009 through Late 2011 – High Sierra is the end of the line
• Late 2012 through Late 2013 – Catalina is the end of the line
• Mid 2014 – Big Sur is the end of the line
• Late 2015 – Monterey is the end of the line
• 2017 – Ventura is the end of the line

The 2019 models can't run High Sierra, so we know you don't have one of those.

How to download and install macOS - Apple Support

If you are eligible to upgrade to Catalina or later, and do so,

• 32-bit applications will stop working
• iTunes will go away, replaced by separate applications

Bummer.

If you want help, you’re going to have to be a bit more descriptive…

I've got 10.13 in a VM, and when I try to sign in to the App Store nothing actually happens. Apple have recently changed things and in Sierra there's an actual error message.

If the OP is running 10.13, it's likely it is not possible for them to update to anything higher.