You may want to restart your router and modem to fix this problem.

You may have a slow internet connection.

It is possible that your Mac may unable to reach the update server.To reset PRAM, turn off your Mac and then turn it on and immediately press and hold the Option, Command, P, and R keys together for about 20 seconds.

Reset PRAM on your Mac and then try again.If you have any external hard drives or peripherals connected to your Mac, try removing them.If the update does not complete, your computer may seem stuck or frozen, for an extended time, try to restart your computer by pressing and holding the power button on your Mac for up to 10 seconds.Also ensure than your Macbook, if it is a Macbook, it is plugged it into AC power. Do not put your Mac to sleep or closing its lid. Make sure that you have an Internet connection to update your Mac.If you see a “ Update Not Found” error, see this article.You can easily free up storage space on your Mac. To see if your computer has enough room to store the update, go to the Apple menu > About This Mac and click the Storage tap. Make sure there is enough space to download and install an update.There could be a variety of reasons for this. It is important that you back up your Mac before you do anything. Please try the tips belove if you are unable to update your computer:

Sometimes, you may see an error message saying “The macOS installation couldn’t be completed” or “macOS could not be installed on your computer”. By stalled, we mean that macOS updates won’t download and install successfully. Some Mac users, unfortunately, have been and continue to be affected by various macOS software update issues where they may experience a stalled macOS update. More specifically, the focus of this article is on what you can do to when your Mac won’t update to the latest version of macOS.

This article explains how you can fix the software update problem.

Update mac operating system