I'm unable to get software update. What should I do

I think I can help you. It is important to understand the difference between updates and upgrades. An update is when we install a newer version of the same operating system. It will be found in the System Preferences by clicking the Software Update button. If you already have the latest version installed, you will be notified that no update is available. An upgrade is when you install a newer  operating system such as upgrading from Catalina to Big Sur. What you can install is dependent on the year and model Mac you have. Upgrade links may be found in the Apple App Store. I hope this information is helpful in clearing up any confusion.