We have WPMU (WordPress Multisite) support on our theme, so if you want to use it, first you have to follow this instructions to enable the Multisite functionality for your website.
After Enabling the core functionalities for multisite options you have to prepare the ACF fields for each new site you have created except the main site.
Here is the steps, you have to do to use ACF at child sites.
1. go to child site’s dashboard
2. go to Custom Fields ▸ Tools
3. click the Choose File button, select the acf-fields.json file (download it from here) and click the import button