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…

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

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.