mac os Monterey

There are factors:

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

5 - Running behind a VPN which may increase Packet losses

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

If downloading Monterey from behind a VPN it may increase Packet Looses and an incomplete or corrupted Installer.

Any Third Party Applications that will interfere with the normal operation of the OS, alter, modify, remove or delete or attempt to do so is an invitation for disaster and may require a Reinstallation of the OS.

This includes AntiVirus, Disk Cleaners, Disk Optimizes, UnInstaller etc.

The The Built in Security  is all that is required.

How much free space on the disk? You need 40-50 GB free.

Try booting to Safe Mode, which takes minutes, and doing the download and install.

How to use safe mode on your Mac - Apple Support