In what template file are you including the acf_form() function?
What post type are you trying to create?
If you are trying to create a new “Post” then the function needs to be in the single.php file, or in one of the template parts used by this template. If you’re doing this in some other template then you’re going to need to supply the field groups that need to be included as part of the arguments to acf_form() https://www.advancedcustomfields.com/resources/acf_form/
