Seriously, I dislike MS servers. That error message is useless, like all MS errors. I’m not familiar with your server setup so I can’t really advise you on how to get it to display the real PHP errors that will give you useful information. I’d start by contacting the host and asking them.
As far as the access goes, debugging other peoples site’s or doing dev work on them is not something I do here. This is a user forum, users helping users.
As far as insert post, yes, since you’re running you function before ACF does you may need to insert the post first, when inserting the post there are some other requirements https://developer.wordpress.org/reference/functions/wp_insert_post/, like inserting content because it’s required.
