i mac will not allow upgrade to latest version of os, gives no reason

Is it a 2015 or later model (from Apple menu > About This Mac)? See compatibility section of How to upgrade to macOS Monterey - Apple Support

Doe it have enough free space? See the following from the same support article:

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.