downloading the installer build manifest failed

1. Have you tried downloading from both the App Store (search for macOS Monterey) and from Software update in System preferences?
2. If you have any antivirus or "cleaning" apps uninstall them using the app's instructions.
3. Boot to Recovery and repair/First Aid the startup disk. You can't repair/First Aid the disk you booted from.

About macOS Recovery on Intel-based Mac computers - Apple Support

If disk OK then d boot to Safe Mode, which takes minutes and redownload the installer and try again.

https://support.apple.com/en-us/HT201262

4.If you have not fully update the existing macOS version they fully updating and try again.

5 Open the Terminal app and enter this command

softwareupdate –list-full-installers

This will give you list of available downloads. Then enter this command to download an installer:

Then enter the command:

softwareupdate --fetch-full-installer --full-installer-version xx.x.x

where xx.x.x is the version number in the list you wanted. You will likely enter the value 12.1 for current 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 and may corrupt 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