good cause i'm having the same problem so i'm happy you found a solution. I can't find where to click buisiness settings in facebook
**bleep**, this actually worked for me. THANK YOU SO MUCH!!!!!
I've figured it out after some trial and error.
Go to business settings -> Accounts -> Pages
Select the page
Select the admin person linked to the account
UNCHECK manage jobs and view earnings insights
CHECK ON admin access manage page
I had all options checked and this was confusing it and giving me an access right of Custom instead of Admin.
If you uncheck the two items above, it will set you as page admin and you'll be able to connect to your page via Shopify.