Suppose, you have set up Multisite and use get_field() function (Activated ACF Plugin) in your header.php file to get custom filed content.
This has no issue for normal site without network but, for Multisite, when a new user is registered, the user gets an activation email with a link to YOUR-SITE-URL/wp-activate.php?key=RANDOMKEY. This link however turns out blank, without being able to access the get_field function. To display activation link content, you can wrap the call to get_field in an “if function_exist(‘get_field’)” function, which solves your issue but this obviously, can’t load the contents of custom fields.
To fix the undefined function issue, load acf.php in your function.php .