I Can't install the Monterey I downloaded it and free some space more than 16 GB and I can't install it also

If upgrading from macOS Sierra or later, macOS Monterey requires 26GB of available storage to upgrade.

If upgrading from an earlier release, macOS Monterey requires up to 44GB of available storage.

from:

macOS Monterey is compatible with these computers - Apple Support

You need to,create more space on your drive (9 GB) and, you need to maintain at least 15 GB of empty space at all times for the OS to function properly. so, you need to delete or move more than 20 GB of files, videos, photos. You need to manage your hard drive - it is unbelievably small.