Mac Software Update Download Failed. Now browse and choose the software update option. If your Mac update failed definitely owing to the storage space the only thing you need to do is to free up the space on Mac.
Select the device icon on iTunes. This usually fixes the issue as you no longer depend on Apples servers to download the firmware file. So if your Mac computer almost ran out of the storage space your macOS update stuck at the beginning for sure.
But you can also manually download the updates.
If the above steps failed then consider updating it manually using iTunes. If you are on Catalina you might think you have to stick to downloading the new software via Software Update but you can still grab the download via the Mac App. In the Office for Mac updates area click the update version of the product that you want under Available Downloads. If the above steps failed then consider updating it manually using iTunes.