Hello and welcome.
Do you know if they installed any Mobile Device Management (MDM) or security software on the Mac? If so, you may need to have them uninstalled and/or erase the Mac completely.
That model does support macOS 12.5 Monterey. Try looking in the App Store application and search for Monterey. You should then be abel to download and install it when signed in with an Apple ID. As a side note, some older software versions may not run under Monterey, so you may need to update or pay for an upgrade to the latest versions.
How to upgrade to macOS Monterey
Also, it is strongly recommended to have a Time Machine or other backup of your data before upgrading. You will need at least 26 GB of free space, since you are running macOS Sierra currently.
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