1 - Empty Space Requirements >> If upgrading from macOS Sierra or later, your Mac needs 26GB of available storage to upgrade. If upgrading from an earlier release, your Mac needs up to 44GB of available storage.
2 - Safe Boot method attempted as per above
3 - This method may seem unnecessary but sometimes the Router Restart clears Network issues from ISP to Apple Servers
1 - Shutdown Router.
2 - Shutdown computer and disconnect all External Drives.
3 - Restart Router. Connect to Router via Ethernet Cable. If via Wifi - Disconnect all other Devices using the Wifi - want the Maximum Bandwidth only for this computer.
4 - Restart computer in Safe Mode. It does a Repair Disk, Clear System Cache files and only load Apple Software, extensions and Fonts.
5 - Will load slowly - Normal. Once Logged in - open Apple Apps Store and Get the Full Version of Monterey.
6 - It will take some time and several pauses will occur - normal. Countdown clock may say 15 minutes but in Real time may be 30 minutes. Same goes for clock at 10 minutes could be 20 minutes in Real time. Eventually , Full Installer will download and a New Application in Applications Folder will present called Install Monterey.
7 - Stop there and perform a Time Machine Backup.
8 - If desired - have an External Drive - Copy & Paste the Install Monterey Application to external drive for future usage - making a Bootable Installer.
9 - Now run the Installer.