I had that error a couple of times, and it inexplicably resolved itself. Try having the person join again (which may require a new invite). I think only 1 out of 4 members of my family were able to join without that initial error message. This was back when Steam had just switched how they handled family sharing, so I assumed it was just an implementation bug of some sort. One of my friends took three attempts before they could join, but it worked ¯_(ツ)_/¯
Though I will note that steam family sharing no longer works if the person is located in another country for the purposes of Steam billing region (so my Norwegian friend could not join my UK family)
I had that error a couple of times, and it inexplicably resolved itself. Try having the person join again (which may require a new invite). I think only 1 out of 4 members of my family were able to join without that initial error message. This was back when Steam had just switched how they handled family sharing, so I assumed it was just an implementation bug of some sort. One of my friends took three attempts before they could join, but it worked ¯_(ツ)_/¯
Though I will note that steam family sharing no longer works if the person is located in another country for the purposes of Steam billing region (so my Norwegian friend could not join my UK family)