Hello! We are excited to announce Steam Families is now available for all users. Steam Families is a collection of new and existing family-related features. It replaces both Steam Family Sharing and Steam Family View, giving you a single location to manage which games your family can access and when...
I don't really know how it works but according to a lot of other people here it doesn't work unless you are in the same region. This isn't the only person here saying they can't use it because they don't live near their family.
They're doing IP location checks, and they're doing them badly (there's not really a way to do them well). It's not working for me with people in the same town, and other people are reporting it's randomly working or not working with locations in the same neighborhood.
Most of what I'm reading online talks about an error complaining about region, in which case you'd want to make sure you're in the same store region.
Other main suggestion is signing into your steam account on their computer. You could probably use something like Microsoft quick assist (which should already be installed iirc) for that
Good luck, if you get a different error or run into other problems please let me know!