The_field() functions is used to echoes the returned value. Since the post object field returns WP_Post object, you’ll get that error because PHP doesn’t allow you to echo an object. Please take a look at this page to learn more how to show a post object field: https://www.advancedcustomfields.com/resources/post-object/.
I hope this makes sense.
Welcome to the Advanced Custom Fields community forum.
Browse through ideas, snippets of code, questions and answers between fellow ACF users