1 - Like poor Wifi connection
2 - Slow wifi connection
3 - Too many other people downloading at the same time and Apple Severs are overloaded
4 - Software on the computer interfering with the download process including AntiVirus Software
Notation: Disabling the AV Software may or may not solve the issue. Removal of the AV Software may increase the chances of the download to perform as expected.
5 - Running behind a VPN which may increase Packet losses or corruption of the download
6 - If via wifi too many Other devices using the wifi at the same time reducing the Bandwidth to your computer.
7 - Best Results are via Ethernet Cable to Router
8 - Restart in Safe Mode. This will perform a Disk Repair, clear cache files and only load Apple Software, extensions and fonts. The boot up will be slow and can take some time - Normal and Try Again using the Software Update Applet in System Preferences
9 - Upgrade to Big Sur first and then move up to Monterey
How to get old versions of macOS and verify what version this computer Qualifies to install. For Best results use Safari to commence the download as Others may not work.
Safari will open a redirect and open the Apple Store where the download can commence.
Computer Qualify for the Monterey upgrade
macOS Big Sur 11 hardware requirements