i cant update my mac

A late 2013 iMac can be upgraded to macOS Catalina or earlier. See the following from Identify your iMac model - Apple Support

iMac (21.5-inch, Late 2013)

Model Identifier: iMac14,1

Part Number: ME086xx/A, ME087xx/A

Newest compatible operating system: macOS Catalina

To upgrade, see information in See: How to download macOS - Apple Support

Be aware that if you upgrade to Catalina 10.15 you will also need to update any non-Apple 32-bit apps. See: https://www.macworld.com/article/232735/how-to-check-if-mac-software-is-32-or-64-bit.html

What happens when you use Safari and click on Catalina (or an earlier version) in the "Or use the App Store or your browser" section of How to download macOS - Apple Support ?

Someone here may be able to help if you can provide more details about how you are trying to do it and what happens when you try that.