There are no solutions for this at the current time.
This has been requested a lot. I understand the logic but Adding a new post from the relationship field is not something that’s going to be easy to do and I don’t think it’s something that the developer will add.
Here’s the problem is, you’d need to create an entire WP post editor inside the relationship field or you’d just get a post with a title.
Would you want to create a new related post without adding the content that the post supports? Setting categories and other taxonomy values?
If the developer created a way to just add a post and all that was added was the title then the next request would be to add a way to add content and it becomes a rabbit hole that has nothing to do with what ACF is designed to do, which is manages custom fields.