I recently went on trying to delete my old Facebook posts and the amount of hurdles and steps necessary and errors involved leads to the thought that without doubt Facebook does not want you to do this.
In order to delete the posts you have to manually click on each and every one and then click on delete. Sometimes it throws an “unexpected error” and then you have to click on everything again, but then it often throws the error again.
Another way is to “mark all” but that only marks 50 posts at once. If you scroll down then it adds another 50 posts, at the cost of about 20 seconds per scroll, so when you scroll down far enough you can accumulate a lot of posts that will be available for “mark all”. But, mind that Facebook doesn’t like you to batch remove all of your history. So after 150 or 200 posts (the amount is changing apparently randomly) accumulated and clicking on delete, they throw a “Challenge required” error. With trial and error you find out that this error appears always after you accumulated too much, so you’d naturally think, let me just go back one step and scroll a couple of time less to generate a new and smaller list. But it’s not possible to generate that list of posts again for some reason, as going away from the activity monitor and back into it again just shows again the >200 posts list nicely prefetched for you. I found the only way to “reset” this view is to log out of Facebook and log in back again, then the activity monitor starts at 50 posts again.
Hopefully when you then batch delete 150-200 posts, there isn’t a special post in between which brings another error when trying to delete from the activity monitor in batch or manual. It seems that this error only appears when you deleted too many posts in a specific time frame. What you need to do is to find that post in the haystack of at minimum 50 posts, click on it’s time such that you get on the posts individual page (better open in another tab or you might get insane eventually, if not already) and then delete the post from there by using the menu button.
Hopefully it allows you to delete and you can continue to batch delete the rest in batches of 150-200. Otherwise you will have to wait for a couple of hours you receive a new delete allowance.
Lastly, even if you at first think posts were deleted, you might have to revisit and see again, as some posts just reappear again. You don’t find them necessarily inside the activity monitor again but e.g. when you use the search, or even worse, when others use the search on these posts.