Try rebooting the computer. A simple reboot can fix a lot of issues. If this does not work, then try booting into Safe Mode to see if you can download the installer.
Here is an Apple article with links to the various macOS installers:
How to download and install macOS - Apple Support
If you already have the Monterey installer downloaded (or part of it), then maybe it is corrupt. Try deleting the Monterey installer app from the Applications folder and empty the Trash. Then try downloading the Monterey installer again.
Another option is to boot into Internet Recovery Mode using Command + Option + R to access the online macOS Monterey installer. Unfortunately some Macs may instead just boot to the online installer for the OS which originally shipped with the Mac from the factory. You need at least 50GB+ of Free space for this option (more than if you download & run the installer from within macOS).