In the last two or three days, I cannot on my iPhone share to my Facebook timeline. However, I can share to other groups.
When I post, I see the various options where I can post, but my timeline shows a beach all cursor. Attempting to post just says connection taking longer than it should.
Restart your phone. This bug started for me with iOS 14 and still exists occasionally. For me the only fix I’ve found that works reliably is to restart the phone.
Can you just do it within the FB app? If it’s a photo, you should be able to access it from the app. If it’s a web page, share / copy and then paste into FB.
One other thing to try is to log out of FB from within the app and log back in again, see if that helps.
I’m using various apps like Apple News and Photos and using the share sheet. If I go into Facebook either the app or webpage, I can type in the URL of the page or download the photo. However, it’s much easier to use the share sheet.