I have been having the same problem for over a month now. I had changed my @username for my facebook and instagram, and now shopify won't recognise that I am the owner and admin of my page. I own 3 facebook pages under my facebook account. The one I need to connect is not recognised. The other 2 pages I have work. The one I need, used to be connected but reconnecting it no longer works. I can click on it, when I am initially prompted to log into my personal facebook account, it is not greyed out. But when I select only that page it says that I am not the owner or admin of the page. If I select all 3 of my pages then only 2 appear in the drop down menu. Not the one I need.
The same thing happen to me today, without reason the Facebook have been disconnect from shopify and then when im trying to reconnect, it's told me there is no page found, if i have no page i can create one. But of course i've got one and it was working fine before. I have try to follow your tutorial but isnt working for me, anyone else got the problem ? It's havebeen solved ?