Multisite not working

Not working on wp multisite.
Trying to add field in one site and get js error:

init simple_fields_file_field
http://site.com/wp-admin/admin-ajax.php Failed to load resource: the server responded with a status of 500 (Internal Server Error)
http://site.com/wp-admin/options-general.php?page=simple-fields-options&action=edit-field-group&group-id=2 Failed to load resource: the server responded with a status of 500 (Internal Server Error)

https://wordpress.org/plugins/simple-fields/